We’re still in the first inning of the social media game, yet the urge to pick winners is strong. Anyone who’s trying to make sense of all the activity right now is being whipsawed. A year ago, MySpace was all the rage, then YouTube took center stage last fall. Early this year, everyone was atwitter about Twitter and now Facebook is growing like kudzu to the applause of investors and the press.

While there will no doubt be other market darlings, I think Facebook is the first of these nascent communities to deserve serious attention from b-to-b marketers. If you haven’t been paying much attention, you might still think of Facebook as the social network for college students. In fact, as recently as last fall, a personal still needed a “.edu” e-mail address to join.

All that changed last late year when Facebook made two critical decisions: it opened membership to anybody who wanted to join and it permitted third-party software developers to build applications specific to the Facebook platform.

The results have been astonishing. Membership has doubled since the first of the year, eclipsing 30 million in early July. What’s more interesting to marketers is that the demographics of this member base are intriguing. As Rodney Rumford points out in this analysis, members over 25 years of age now account for half of Facebook traffic. That’s remarkable when you consider that most of those members couldn’t even get to the site 10 months ago.

An even more telling statistic is audience engagement. According to Comscore, 93% of Facebook members log on at least once a month and 60% use the site daily. Those are impressive figures for even a small community site; for one with 30 million members, they’re mind-blowing.

If you register on both MySpace and Facebook, the differences will whack you in the face. MySpace’s heritage as a music site makes it feel at times like a giant virtual nosh pit. Member pages are festooned with graphics and music plays helter-skelter. In the year I’ve been a MySpace member, I don’t think I’ve received a single message from someone I knew.

In contrast, when I registered for Facebook, I was flooded by invitations to become friends (social network lingo for establishing a connection) with dozens of current and past colleagues. Facebook allows you to monitor some of the activities of your friends, and it’s an impressive display to watch. People I know are busily exchanging software applications to recommend books, movies, travel destination and professional web sites. The Society for New Communications Research, of which I am a member, chose Facebook as the community of choice for its professional members. And I continue to get “friends” request from actual friends almost daily.

If it keeps up this momentum, Facebook has the chance to succeed where earlier professional networks like LinkedIn didn’t. While LinkedIn has some valuable professional networking features, it has the feeling of a software application more than a community. Facebook’s approach to the market is proving to be more effective: it started as a community site and then added networking features. Its roots as a gathering place for college students has helped it to continue to attract the kind of members that marketers want to reach. If it continues to grow at its current rate for another year, it will reach the status among adult professionals that MySpace enjoys among teenagers: you simply have to be there.

This is not to say that Facebook is perfect. Its closed e-mail application doesn’t sit well with people like me, who live in their inboxes. Some of its distinctive metaphors — like writing graffiti on someone’s wall — can be confusing to new members. The process of creating a new group can also be somewhat cumbersome and confusing. And while its applications are impressive, Google still delivers better quality and features overall.

Nevertheless, business marketers should become familiar with Facebook. It has a chance to become the gold standard for professional networks. Even if it fumbles the opportunity, the dynamics of what’s going on there are important to understand.

Update: Maggie Fox just passed along this press release from Comscore, showing Facebook traffic up 270% year-over-year, compared to MySpace’s 72%. Of course, MySpace started from a much higher base and is still the leader overall by a wide margin, but Facebook is closing the gap.