Instant messaging startup Meebo has opened up its online service to outside developers. It has become — wait for it — a platform.

In its new guise, Meebo will allow hand-picked developers to put their browser-based applications in front of the site’s 6 million monthly unique users. The widgets will exploit IM technology to focus on instantaneous interactions between the folks on your buddy list. The first applications announced by Meebo include audio chat, video chat and a video broadcast service that allows you to stream video to all your friends at once. (Think of it as your own live TV show.) Other buddy-friendly apps in the offing include a digital map mashup that could be accessed simultaneously by multiple users. You could debate the merits of a particular bar or restaurant, for example. Games are another obvious category, Battleship anyone?

Meebo CEO Seth Sternberg is quick to point out that he and the Meebo co-founders had this platform idea two years ago when they were first pitching the Mountain View, Calif., company to investors like Sequoia Capital. While that may be true, Facebook did it first, and is effectively the largest social network platform on the planet these days. And don’t forget that other massive social networks like MySpace and Bebo are busy working on their own platform plays. The big question is, is their room for more than one, even a handful of these platform plays?

“What it comes down to is that an attractive platform is directly related to the number of users it can bring to the developers,” says Jeremy Liew, a partner at venture capital firm Lightspeed Venture Partners. “If that’s the case, not everyone can be a platform.”

Facebook and Meebo are already competing for users’ time. Now they will be competing for developers’ attention and efforts. Meebo’s tack, and it’s a smart one, is to pay developers more to do their coding for Meebo. The startup will share with the developer half the advertising revenue any application generates. And rather than let developers hawk ads, Meebo will do the selling for the site. Facebook leaves ad sales to the outside developers, and while a few of the most popular Facebook applications make money for their creators, the vast majority do not. “These people are engineers,” Sternberg says. “They’re good at coding applications, not selling ads.”

The Meebo gang also claims their service is not a direct competitor to Facebook’s. “Facebook is all about leaving a message for someone to pick up later, it’s asynchronous,” says Sternberg. “We are all about live, synchronous communication. It’s very different and those developers that have tried it on Facebook have not found an audience.”

Meebo is welcoming those developers to their side of the social network playground. And there is reason to believe if Meebo can offer them the right kind of distribution and the right kind of remuneration, they will take that offer. Popular widgetmaker RockYou, for one, sees the opportunity in developing its multimedia slideshows and other social networking widgets on top of Meebo.

“People haven’t thought that much about IM because the software has always been proprietary,” says RockYou co-founder Jia Shen. He notes there’s no reason why Meebo can’t become as successful a platform as Facebook. Unlike the social network, “Meebo is going to be more focused on real-time interaction -– that’s its advantage.”

Still, there is nothing to prevent Facebook from developing its own IM service to try to keep its members and all the developers to itself. Or not bother and just buy Meebo, says StumbleUpon CEO Garrett Camp. “They’ve got to do something with that $250 million they just got.”