While his Web 2.0-era peers sold their companies and founded new ones, Garrett Camp stuck with StumbleUpon for 10 years since creating it as a grad student. That included selling the company to eBay for $75 million, buying it back, and notching 25 million registered users as of last month.

Ten years seems about long enough, Camp said today. “It was my first job and my first love,” he explained.

Camp hasn’t been exclusively focused on StumbleUpon. He co-founded the on-demand ride service Uber and is chairman there as well.

Camp said he’s been thinking about stepping back for years, and more seriously in the past six months. I asked if he wouldn’t have rather quit when Uber was getting started a few years back.

Camp said no. He does have a few new start-up ideas he’s thinking about, but they would be “virtual” products, unlike the real-world challenge of Uber. Camp said Uber CEO Travis Kalanick is better suited to that operational role, where Camp wants to work on products.

StumbleUpon co-founder Geoff Smith is to stay in his position as CTO.

The company is rolling out a newly redesigned Web site that features a new logo, new colors and an integrated “Explore Box,” or search engine, that had previously only been available in beta.

StumbleUpon has also signed up 250 partners for channels on the site, which will act as verticals users can “follow” in order to get the interesting content they want. The partners include such Web sites as Yelp, Gilt Groupe, Vanity Fair and Funny or Die, as well high-profile names like actor Jim Carrey, athletes Mariano Rivera and Paul Pierce, and late-night TV host Chelsea Handler.

While StumbleUpon is getting a face-lift and adding some boldfaced names, it isn’t changing any of its back-end technology: Users will still “stumble” from site to site, which will be served up to them based on StumbleUpon’s algorithm that factors in interests, likes and your friends’ interests.

StumbleUpon founder and CEO Garrett Camp said the redesign was spurred by feedback the company was getting from users in focus groups. Basically, while the users liked the site’s signature stumbling action (which I previously called a procrastinator’s friend and insomniac’s dream), they wanted easier ways to follow their favorite brands and content.

“Some of the words we used when describing StumbleUpon were surprising, adventurous, exciting, and when we put our logo in brand in front of test users, they weren’t saying that,” Camp said. “We wanted to make it that for them, while also simplifying the site.”

StumbleUpon launched in 2001 as a way for people to find interesting content on the Web. In 2007, the company was acquired by eBay for $75 million. Then, in 2009, Camp, his co-founder Geoff Smith and other investors bought the company back and took it private again. The site went through a minor refresh then, but these new updates mark the first major visual changes to StumbleUpon since it was created. The company currently claims 20 million users and more than 1.2 billion stumbles per month.

While recent data showed that StumbleUpon is now the biggest referrer of traffic to other U.S. Web sites — beating out even Facebook for that title — the changes come as giants like Google and Facebook are dominating the Web ad space, with other Web services clawing for more market share, as my colleague Peter Kafka reported earlier. StumbleUpon’s entire revenue model is advertising — around 3 to 5 percent of all stumbles will land on an ad — and the company is uncertain whether these new celeb channels will end up being new ad space.

It’s the Wild Wild Web out there, kids.

What’s a little odd about the situation is that we wrote only two months ago about StumbleUpon’s new VP of engineering hire, which at the time was Balogh (pictured at right).

Balogh, it turns out, never got to StumbleUpon. He did the interview about the new gig with AllThingsD after agreeing to join, but then decided to take another offer to be VP of storage infrastructure products at Google, where he is now. (Balogh declined to comment on the situation.)

Meanwhile, Tulsi will report to StumbleUpon CEO Garrett Camp, who previously led engineering himself. Tulsi will focus on scaling, advertising and mobile projects.

Tulsi (pictured left) was persuaded to join StumbleUpon after six years at Google spent on products like Google Analytics and YouTube. He said in an interview this week he was particularly interested in working with StumbleUpon’s “strong co-founders,” who sold their company to eBay but then bought it back and continue to be engaged.

Plus, it’s working; StumbleUpon now has 15 million users and gives one billion content recommendations per month. According to StatCounter, StumbleUpon has been referring more traffic to other Web sites than Facebook since mid-June.

And StumbleUpon feels like home, in a way. Tulsi is the 20th former Googler at StumbleUpon, and there are only 80 people at the San Francisco-based company.

Tulsi, by the way, actually did start on Tuesday, so it seems unlikely there will be any funny business this time around.

Balogh said in an interview that his projects will include “a continuing and unrelenting focus on the recommendation technology,” plus improving features that indoctrinate new users and engage returning users, as well as extending StumbleUpon’s development of the open-source Hadoop database HBase.

Balogh had left Yahoo last April, citing “personal priorities” and a specific family situation.

He said that those issues were resolved last summer, and around the beginning of this year he started looking at various technical and operating roles at both larger companies and start-ups.

Balogh will join StumbleUpon June 1, reporting to StumbleUpon CEO Garrett Camp.

StumbleUpon previously did not have a similar role, with its engineering team reporting to Camp directly. With 70 employees, that was getting understandably unwieldy.

But Balogh said he appreciates that StumbleUpon is small compared to where he came from. “It’s not like Yahoo where there’s a huge chain of platforms,” he said. “It’s a rapid delivery model.”

StumbleUpon has 15 million monthly users and one billion recommendations per month. It recently raised $17 million to support continuing growth after being spun out of eBay in 2009. eBay had bought the company back in 2007 for $75 million in cash.

In some ways, discovery is the opposite of search. About 50 percent of searches on YouTube are “broad,” according to YouTube Director of Product Management Hunter Walk, by which he said he means users are seeking an experience rather than a particular video. So YouTube’s intent with recommendations is less about one right answer and more about a cluster of answers, or eventually, a narrative.

One way to measure the success of recommendations is to ask users to rate them directly, which is a major component of Pandora’s and StumbleUpon’s systems. Pandora, for example, has eight billion thumbs-up and thumbs-down actions in its index. StumbleUpon has an 80 to 85 percent thumbs-up percentage, said StumbleUpon CEO Garrett Camp. Users look at a page they have stumbled upon for an average of about eight seconds, and a median of 20 seconds, he said.

The social graph is only a small part of good recommendation engines, Conrad, Walk and Camp agreed on a SXSW panel about content discovery. Rather, a recommendation engine is a mix of art and science hinged on refining small changes over time and understanding how users respond to them.

I moderated the panel, so I’m sure I missed writing down all sorts of interesting tidbits, but I wanted to share some of the insights and stories.

In addition to the plaid-shirted trio of Conrad, Walk and Camp–speaking on music, video and Web page recommendations, respectively–we added Google Hotpot product manager Lior Ron to the panel after bumping into him in the hallway. (Hotpot is Google’s newly launched personalized local recommendations site.)

Walk warned to be careful with analytics, saying that positive stats about recommendation performance don’t necessarily correlate with a good user experience. As YouTube has improved its recommendation algorithm, it has negatively impacted the number of video playbacks it gets.

That’s because better recommendations reduce the number of times people start watching and then skip a video, Walk said. Yet the company has almost tripled session length in the last few years as it has reduced these skips, and recommendations have helped improve the overall experience.

One of the most significant improvements YouTube has made to its recommendation technology, Walk said, was in messaging and presentation. When the company started explicitly stating why it was recommending a video–for example, you should watch this Britney Spears clip because you just finished Justin Bieber’s latest post–satisfaction improved.

As Walk explained, “If it was wrong they didn’t blame us; They blamed themselves.”

Ron said there’s plenty of room for companies to play with recommendations, and that he’d welcome more Netflix Prize-like approaches that might introduce significant improvements. “We’re not living in a world with millions of recommendations and we need to turn it down,” he explained.

But often discovery is about that one right thing at the one right time. Conrad said Pandora’s goal for recommendations is to take the smallest possible signal and provide the best possible personalized results. The Internet radio company A/B tests everything it rolls out to thoroughly measure the impact on user experience.

Pandora has experimented with social–for instance implementing Facebook’s instant personalization feature–but it’s just one signal of many. Conrad said the company quickly realized that it shouldn’t have assumed friends had the same taste in music.

(I can vouch for this awkwardness, having loaded up Pandora while logged into Facebook and gotten a recommendation for a curated Celine Dion channel.)

Camp said that for his purposes, the social graph is too much of a closed loop, taking away from the serendipitous recommendations that StumbleUpon works hard to deliver. The company reserves five percent of its stream for entirely new stuff, he said.

StumbleUpon also doesn’t try to hard to figure out what’s on a Web page in its index, because its point of the service is to deliver a variety, rather than a homogenous stream, of content.

YouTube would like to add features that show “what your friends haven’t watched,” said Walk, so users can have the satisfaction of discovering something rather new, rather than the disappointment of learning after sharing a viral video that they’re the last to know about it.

Ron disagreed to some extent, at least for his domain area of restaurants and other local establishments. Knowing where your friends have gone is one of the more important signals, he said, especially knowing where your friends have gone recently.

One more funny anecdote: Pandora users tend to create channels for Christmas music at that time of year. But the company ran into a problem because there was an indie band named “Christmas.” It was worried that users looking for cheery carols would get an unexpected and dissatisfying stream of random indie music.

But before Pandora could introduce a fix, users took care of the problem by thumbing down those selections, effectively filtering out the Pandora spam (accidental or otherwise) of the band named Christmas.

Images grabbed from the panelists’ profiles on their company sites.

Last week, StumbleUpon announced it was buying itself out of its much-vaunted previous corporate buyout, by being born again as an “investor-baked start-up.”

The Canadian-born social-bookmarking company, which was launched earlier, had come to the Bay area in 2006 and got some fancy angel investors, who ponied up a couple of million dollars. It soon became a traffic-generating hit.

Then, StumbleUpon was bought by eBay (EBAY) two years ago for $75 million in one of Web 2.0′s high points.

End of a fairy tale? Um, nope.

Soon enough, due to both buyers’ and sellers’ remorse, rumors of the San Francisco-based company being sold by its new owners swirled around it, although there was no sale.

Instead, last week, StumbleUpon announced that a roster of well-known Silicon Valley investors, including Ram Shriram of Sherpalo Ventures, Accel Partners and August Capital, would return it to its roots.

Its founders, Garrett Camp and Geoff Smith were included, with Camp as CEO.

In a video interview he did with BoomTown, here’s Camp talking all about the latest shift for his start-up, including discussing some more changes on the site, such as an even more personalized Webification of the content discovery service.

Here’s the video:

The content-discovery service, StumbleUpon, has gotten itself back to start-up status, after being bought by eBay two years ago.

It announced today that it was returning to being an “investor-backed startup” by a roster of well-known Silicon Valley investors, including Ram Shriram of Sherpalo Ventures, Accel Partners and August Capital.

Its founders, Garrett Camp and Geoff Smith are also back, with Camp now in place as CEO.

“We are grateful to eBay for its guidance. However, we realized there were few long-term synergies between the two businesses. It is best for us to part ways and focus on our respective strengths,” said Camp, stating the very obvious.

That’s quite a boomerang since it was acquired by the auction giant in 2007 for $75 million.

Before that event, which was at the height of the Web 2.0 fervor, the Canadian-born social-bookmarking start-up, which launched several years ago, came to the Bay area in 2006 and got some fancy venture investors (Mitch Kapor, Ron Conway, Shriram and others) who ponied up a couple of million dollars. It soon became a traffic-generating hit.

But rumors of the San Francisco-based company being sold by eBay (EBAY) have swirled around it almost since it was bought, although there was no sale.

The same has been true for eBay’s other purchase, of voice-over-IP service Skype. A recent report in the New York Times said its founders were also considering buying Skype back from eBay.

Under eBay, the site has floundered a little bit, but made some changes, such as unveiling a new Web-centric look and feel and a new partnering program last fall that represented a major shift for the online discovery service.

In that change, users no longer had to register for the service or download its toolbar to “stumble” the Web.

Terms of the deal were not released, but we’re digging! Um, stumbling!

More to come, but here’s the full press release from the company, as well as a video I did at the party StumbleUpon threw after getting acquired by eBay, including an interview with then-thrilled Camp:

StumbleUpon Goes Independent; Backed by Founders and New Investors

April 13, 2009 – StumbleUpon, the best way to discover new content on the Internet, today announced that after nearly 2 years as a subsidiary of eBay Inc., it has returned to the ranks of an investor-backed startup. StumbleUpon is now backed by the original company founders, Garrett Camp and Geoff Smith, as well as a number of well-known investors including Ram Shriram of Sherpalo Ventures, Accel Partners, and August Capital. Camp takes on the role of CEO of StumbleUpon.

“We are grateful to eBay for its guidance. However, we realized there were few long-term synergies between the two businesses. It is best for us to part ways and focus on our respective strengths,” said Camp. “This change makes it possible for StumbleUpon to continue to innovate and focus on becoming the Web’s largest recommendation service.”

“StumbleUpon helps users discover the best of the web–it’s a way to find interesting content you wouldn’t think to search for,” said Shriram. “StumbleUpon’s personalized recommendation engine brings serendipity back to websurfing, and lets users sift through socially-endorsed content with a single click.”

StumbleUpon will remain focused on helping people discover interesting content by increasing the accessibility of the StumbleUpon service and the quality of recommendations. In addition, StumbleUpon has plans for several new products and features to be released in the upcoming months.

While rumors of its impending re-sale have apparently been greatly exaggerated, what’s true about StumbleUpon is that its new Web-centric look and feel and a new partnering program represent a major shift for the online discovery service.

The San Francisco-based company, which was founded in 2001 and sold to eBay last year for $75 million, is announcing tonight that users will no longer have to register or download its toolbar to “stumble” the Web.

Users can now simply start on StumbleUpon’s site, for example, and stumble all over the Web using their Web browser as guide rather than a toolbar.

The move is being made simply because most Internet users are increasingly loath to install Web plug-ins like toolbars, a requirement that naturally has slowed the growth of StumbleUpon’s service over time.

Currently, StumbleUpon has about six million registered users, although only a fraction of those are responsible for the approximately 12 million daily “stumbles,” all using a toolbar.

“We wanted to attract users who do not want to use a toolbar, making it easy so they could use the service right from the get-go,” said Garrett Camp, co-founder of StumbleUpon, in an interview with BoomTown earlier today.

Camp noted that that the toolbar–which has been downloaded between 11 and 12 million times–has seen that growth slow over time. Nonetheless, it is not being eliminated either.

“[Toolbar adoption] was still growing, but not accelerating,” said Camp. “Being able to stumble without one was the biggest feedback we got from users.”

Along with the Web-stumble change, StumbleUpon is also unveiling a redesigned homepage–see an example of it below; click on the image to make it larger–which is an attempt to make it more of a destination.

With the new look, visitors can find content by topic and more related to interests. Other changes include a new look for profile pages, as well as user reviews, rating and comments.

Along with its distribution shift and site renovation, StumbleUpon is unveiling a partner program called StumbleThru that will allow visitors to discover content within those sites without going to StumbleUpon.

Sites–starting with HowStuffWorks.com and the HuffingtonPost.com and followed within weeks by RollingStone.com and National Geographic–will display a StumbleUpon “badge” or custom widget.

It is not unlike similar buttons that now dot Web pages from news discovery services like Digg, which users can click to find related pages.

Essentially, much as Google (GOOG) delivers custom search within Web sites, StumbleUpon is offering custom surfing, giving publishers StumbleUpon technology to allow its users to surface content within their sites that is often deeply buried.

As to the blog reports that eBay (EBAY) had put StumbleUpon up for sale after owning it for a little more than a year, Camp essentially dismissed them, noting that the unit is still operating as an independent subsidiary of the auction giant.

“They have given us a lot of runway,” said Camp.

Here is the new front page of StumbleUpon:

Also, here is a video I did last year at the exceptionally noisy (sorry!) party that StumbleUpon threw after it was sold to eBay a little more than a year ago:

The Canadian-born social bookmarking start-up, which launched a few years ago, came to the Bay area last year and got some fancy venture investors (Mitch Kapor, Ron Conway, Ram Shriram and others) who ponied up a couple of million dollars.

And, in your typical dot-com Cinderella story, it then proceeded to sell itself to eBay for $75 million recently, after quickly growing its user base to more than two million.

Thus: Party on, Garth!

Here’s a little video I did last night, talking to one of the impossibly young co-founders, Garrett Camp, and also StumbleUpon’s “businessman” (a code word for biz dev), David Lee.