The New York-based company is still closing the round, but I’m told existing investors Bessemer Venture Partners and Spark Capital will re-up, and that Softbank Capital NY will also come aboard. There’s at least one other significant investor likely to join, but I don’t have their name (feel free to tell me, though–I keep all correspondence anonymous: peter@allthingsd.com).

OMGPOP has raised $6.5 million during its four-year history, which saw it start out as a sorta-dating site. But for the past few years it has focused squarely on free-to-play casual games, and has recently been trying to generate revenue via a freemium model, as well as some forays into advertising.

And like many casual games start-ups, it is getting a big boost by integrating its games into Facebook. It integrated with the social network this summer, and CEO Dan Porter says that move has doubled the company’s user base from three million to six million. Porter declined to talk about funding news.

Interest in casual games has sparked a flurry of deals in the past year, including Electronic Arts’ purchase of Playfish, Disney’s purchase of Tapulous and DeNA’s purchase of Ngmoco. But the big player remains Zynga, which is both tightly integrated with Facebook and often described as an obvious IPO candidate.

Alas, the Foursquare conference, hosted by the Quadrangle PE fund, is an invitation-only affair. And the event, which kicks off tomorrow, is closed to the press except for reporters onstage to interview the stars. And those conversations don’t get released to the public.

That’s a particular bummer this time. Because the Quadrangle guys–who have had a rough year–had the foresight to get a lineup that includes GE (GE) CEO Jeff Immelt, who appears to be in the final stages of selling NBC Universal to Comcast (CMCSA) CEO Brian Roberts, who will also be onstage. Sure would be nice to hear what they say.

Another panel that piques my interest, if only because of the title: “Are Popularity and Profitability Correlated?” It features Twitter’s Stone, LinkedIn founder Reid Hoffman and YouTube co-founder Chad Hurley. And would-be moguls are represented by a start-up pitch panel that includes Dennis Crowley of Foursquare, the superbuzzy mobile service whose name has nothing to do with Quadrangle’s conference.

So once again, here’s the complete list of those you won’t be hearing from this week as they gather at the Plaza in Manhattan. Unless, perhaps, one of my more ambitious colleagues sneaks in–I’m thinking of you, Bobby MacMillan–and gets us a first-hand account.

2009 SPEAKERS
EMILIO AZCÁRRAGA President, Board of Directors and CEO, Grupo Televisa
DENNIS CROWLEY Co-Founder, foursquare
BARRY DILLER Chairman and CEO, IAC; Chairman, Expedia, Inc. and Ticketmaster Entertainment, Inc.
BRIAN DUNN CEO, Best Buy
CHARLES FORMAN Founder, OMGPOP
REED HASTINGS Founder, Chairman and CEO, Netflix
REID HOFFMAN Executive Chairman and Founder, LinkedIn Corporation
CHAD HURLEY CEO and Co-Founder, YouTube
JEFF IMMELT Chairman and CEO, GE
PAUL JACOBS Chairman and CEO, Qualcomm Incorporated
OLLI-PEKKA KALLASVUO President and CEO, Nokia
JASON KILAR CEO, Hulu
LESLIE MOONVES President and CEO, CBS Corporation
ANNE MULCAHY Chairman, Xerox Corporation
JAMES MURDOCH Chairman and Chief Executive, Europe & Asia, News Corporation
BRIAN PHILLIPS CEO and Co-Founder, Thread
DAN PORTER CEO, OMGPOP
BRIAN ROBERTS Chairman and CEO, Comcast Corporation
PAUL SAGAN President and CEO, Akamai
ERIC SCHMIDT Chairman and CEO, Google
IVAN SEIDENBERG Chairman and CEO, Verizon Communications
BIZ STONE Co-Founder, Twitter
HOWARD STRINGER Chairman, CEO and President, Sony Corporation
BEN VERWAAYEN CEO, Alcatel-Lucent
DAVID ZASLAV President and CEO, Discovery Communications

MODERATORS
MARC ANDREESSEN General Partner, Andreessen Horowitz
KEN AULETTA Author and Writer, “Annals of Communications”, The New Yorker
MARIA BARTIROMO Anchor, Closing Bell; Host & Managing Editor, Wall Street Journal Report, CNBC
JAMES CITRIN Co-Leader, Board & CEO Practice, North America, Spencer Stuart
DAVID FABER Anchor, Reporter, CNBC
MICHAEL HUBER Co-President and Managing Principal, Quadrangle Group
BECKY QUICK Co-Anchor, Squawk Box, CNBC
GEOFFREY SANDS Director & Leader, Global Media, Entertainment & Information Practice, McKinsey & Co.
JOSHUA L. STEINER Co-President and Managing Principal, Quadrangle Group
GEORGE STEPHANOPOULOS Anchor, This Week; Chief Washington Correspondent, ABC News

Turns out the majority of those new Twitterers won’t be back in May.

So says Nielsen Online, which estimates that 60 percent of Twitter’s users leave after a month. That makes sense on a gut level to me: Twitter is easy to use, but it often takes a while to make sense, and if you’re not a professional self-promoter–or someone with a lot of friends who are already on Twitter–it may never make sense.

It’s worth noting here that Nielsen is likely overstating the churn because it is only measuring visits to the Twitter.com URL. The majority of Twitter use happens away from the site, on mobile phones and apps like Tweetdeck, and it’s theoretically possible to be an avid Twitterer but never visit Twitter.com after you sign up. I’ve asked the Twitter folks for their take on the stats and will update if they respond.

But let’s assume, for argument’s sake, that the Nielsen stats are correct, or close to being correct. Is that a problem? Obviously, every Web service attracts new users who never come back after they try it out, so churn in itself isn’t a problem. The question is the rate.

The good news is that Twitter’s 40 percent retention rate is higher than it used to be. Prior to the Oprah madness of this month, Twitter’s rate was closer to 30 percent, Nielsen says.

But the measurement company argues, via a fancy chart and equation, that 40 percent retention makes it mathematically impossible for Twitter to achieve significant penetration with Internet users. The simple version is that if Twitter loses three out of five users a month, its growth will be capped at about 10 percent of the audience. Fancy version below (click chart to enlarge):

And here’s how Twitter’s retention rate compares to that of Facebook and MySpace (again, note that Facebook users and MySpace users more or less have to visit the those sites to use them, so the numbers are likely slightly skewed):

So what if Twitter really is a service that appeals to no more than 10 percent of the Internet audience? Is that such bad thing? Not at all. That’s an awfully big number.

And “retention” may end up being the wrong metric to measure a service like Twitter, anyway. See this perceptive post by Andrew Chen (thanks to OMGPOP’s Dan Porter for the link).

But a lot of the Twitter sales pitch–to investors and would-be partners like Google (GOOG) and Microsoft (MSFT)–is contingent on the service’s eventual ubiquity. The appeal of Twitter’s real-time search capabilities, for instance, is less seductive if you’re only searching what a sliver of Internet users are Tweeting about. And knowing that growth is capped could make that impressive hockey stick chart a little less so.

[Image credit: Weegee via the International Center of Photography]

Today’s example: OMGPOP, the casual gaming site formerly known as iminlikewithyou, which has raised a $5 million round led by Bessemer Venture Partners. Previous investors Spark Capital and Ron Conway’s Baseline Ventures re-upped, but not Netscape/Ning/“Project A” investor Marc Andreessen, who took a flier on the company last summer.

The site has raised a little more than $6.5 million to date. A conservative valuation would still make the company worth north of $10 million.

Why is it worth anything? The company, which began life in 2006 as a dating site, has a dedicated user base that logs some three hours a month on the site. But it’s still pre-revenue, as they say.

CEO Dan Porter, who took over for highly visible founder Charles Forman earlier this year (Forman is still at the company, cranking out games), says that will change this spring. He will be selling “premium” accounts/subscriptions, most likely for $5 a month, along with virtual goods (think collectible items, power-ups, etc.).

It’s possible that the site will roll out some kind of advertising at some point–Porter says he’s been testing some ad units with Google (GOOG)–but those would be ancillary dollars (or dimes).

Can that work? Maybe. Casual games–free or low-cost Web games that take little time commitment–appear to be doing well so far in the recession, even as traditional videogame sales falter. And a lot of people are hopeful that the  market for virtual goods–big in Asia, but limited in the U.S. to a handful of games and a few oddities like Facebook’s “gift” items–will finally take off.

Not interested in thinking this one through? Fair enough. Try playing the site’s “Balloono” game, a knockoff of the Nintendo classic Bomberman. I’m waaaaaay older than the site’s core 15-25 demo, but I’ve spent an embarrassing amount of time on this one.