From time to time, we see stories about Facebook losing users in some regions. Some of these reports use data extracted from our advertising tool, which provides broad estimates on the reach of Facebook ads and isn’t designed to be a source for tracking the overall growth of Facebook. We are very pleased with our growth and with the way people are engaged with Facebook. More than 50% of our active users log on to Facebook on any given day.”

The measurement in question was released by Inside Facebook Gold, which takes its numbers directly from Facebook’s advertising tool. Inside Facebook said that Facebook had 149.4 million active U.S. users at the end of May, down from 155.2 million at the beginning of the month, its first such lost in the last year. I asked Inside Facebook writer Eric Eldon to clarify his company’s process, and he showed me how it uses Facebook’s ad targeting tool with the most open parameters possible for each country to get a readout of total potential reach.

Facebook’s last official active user global count was 500 million way back last July, so it’s not as though the company is actively providing the public with better metrics.

A Facebook spokesperson recommended comScore’s traffic estimates, which show light upward movement in the U.S. in recent months.

According to comScore, Facebook had a U.S. audience of 157 million in May, up from 154 million in April and 153 million in March.

In other Facebook news, CNBC reported today that the company has been entertaining bankers at its Palo Alto office to discuss filing to go public as soon as later this year at a valuation that could be more than $100 billion. Which is what has been expected for some time now.

Chart credit: comScore

Please see the disclosure about Facebook in my ethics statement.

The social networking service grew only 1.7 percent in May, Inside Facebook said. While in a normal month Facebook adds 20 million active users globally, the service gained 11.8 million active users in May and 13.9 million in April.

Facebook is still growing overall, but its growth rate was dragged down by actual losses in some markets where Facebook has been around the longest, such as the U.S., Canada and the U.K.

For instance, the U.S. had 149.4 million active Facebook users at the end of May, down from 155.2 million at the beginning of the month, its first lost in the last year.

That’s not a huge loss, and not the first time Facebook growth has appeared to waver, but no loss is good for a company close to going public, and definitely not if it becomes a trend.

Inside Facebook draws its data from Facebook’s advertiser tools and analyzes selections from its premium reports on its public blog.

Facebook is still actively growing in its newer markets, and working on strategic expansion into holdout markets like China (which obviously could be a gigantic and ethically problematic traffic boost), South Korea, Japan and Russia.

But slowing growth is keeping Facebook from reaching 700 million active users per month as quickly as might have been expected. The count at the end of May was 687 million, said Inside Facebook.

We’ll add Facebook’s comment on the matter if it replies to our request.

To be clear, none of these stats count new user registrations. Rather, they’re the number of individual people, including both new and old users, who sign onto Facebook per month.

Update: The company wimped out on actually disputing the reported numbers, but called them into question in an official response.

Please see the disclosure about Facebook in my ethics statement.

But Sittig is now back at the mother ship, having rejoined in January with the title “product architect,” as Kim-Mai Cutler at Inside Facebook reported Friday.

This comes at a time when Facebook has more than 2,000 employees, plans to move to a corporate campus within the next year and says an IPO is likely in 2012. It’s a lot different from when Sittig first joined in 2005.

Sittig had been away from the company only six months before accepting an offer to return. He told NetworkEffect that he felt the outside world lacked the “scale and ambition” of Facebook. Plus, he was lured by some of Facebook’s recent hires: Sam Lessin and Justin Shaffer, brought in through the acquisitions of their companies Drop.io and Hot Potato, respectively (both of which occurred while Sittig was away).

Here’s Sittig’s explanation:

I left Facebook last June because I’d been there for five years and wanted to see what else I could apply myself to. I didn’t expect at the time that I’d head back.

I spent my time off advising companies and looking for new ideas. But for each idea I wanted to build myself, I kept realizing that Facebook was the only place with the scale and ambition where I could build my ideas successfully.

So that, combined with the steady influx of talented people like Sam Lessin and Justin Shaffer, convinced me to say yes when I was approached with an offer to rejoin.

Prior to Facebook, Sittig worked at Napster, after it bought the Mac client Macster that he had helped develop. He studied philosophy at UC Berkeley. Sittig has made angel investments in companies such as Hearsay, which makes corporate social media management tools.

Please see the disclosure about Facebook in my ethics statement.

Facebook VP Dan Rose

Presenting at the webinar will be Myspace CEO Mike Jones–who has been pitching Myspace as “a social entertainment destination” rather than a social network–and Dan Rose, VP of Partnerships and Platform Marketing for Facebook.

Myspace adding Facebook Connect has been rumored for so long you’d think it had already happened, so hopefully there will be more to the deal than that.

If things go as heralded, this will be the first Facebook announcement in a long time where CEO Mark Zuckerberg has not spoken for the company. Dan Rose is a former Amazon guy who joined Facebook in 2006. He has recently spoken on Facebook’s behalf about a partnership with Microsoft’s Bing to give it social data to improve search, and EA’s five-year exclusive Facebook Credits deal. Rose was previously Facebook’s VP of business development and monetization, and according to Kara Swisher was the main negotiator in Facebook’s failed effort to buy Twitter a couple of years ago.

Please see the disclosure about Facebook in my ethics statement.

The Facebook CEO did say there is “almost a guarantee” that the company will double its user base from 500 million to one billion, which is pretty heady stuff, but also pretty hazy stuff. (We already know that Facebook is really big.)

And Zuckerberg again danced around the question of Facebook’s eventual IPO. But unless he flat-out declares that the company will never go public, there’s almost no reason to ask him about it: Facebook is quite clearly headed for an IPO, one day.

Zuckerberg’s summation of Facebook’s global hegemony is worth pondering for a minute, though:

Facebook’s global domination is almost complete, he said. “We are down to just four countries where we aren’t the leading social network.”

Zuckerberg added that in Russia Facebook had just 1 million users, the kind of numbers that saw AOL sell off Bebo and ITV relinquish Friends Reunited. But in Facebook’s case, growth is “doubling every six months”, according to Zuckerberg, and Japan and Korea have similar user bases.

He said that Facebook can tell when an explosive growth “tipping point” is about to be reached by who is “friending” who. When Facebook first launches in a country, nearly all the friend connections are with foreign Facebook users.

“We know that a country has tipped when local-to-local connections outnumber local to foreign,” he added. “It is a long-term thing [and with regard to the four left to tip] we are probably not going to win in six months, not in a year [but] things look promising in three to five years out.”

Meanwhile, if you want a really deep dive with Zuckerberg, check out Inside Facebook’s recent Q&A.