• Fortune, Monday, June 4, 2007 11 AM

It's been a good year for Facebook. Last fall, the social network opened its doors to everyone to make to grow beyond its stagnating user base of around 10 million. Then about 10 days ago, the company, now 25 million strong, gave developers the freedom to use Facebook's platform to either create applications for the social network or to develop their own networks using the company's free tools, a move that has been met with almost universal applause.

Why? Because development and usage have already exploded beyond 300 apps. For example, music network iLike, a competitor to CBS' Last.fm, began offering a plug-in through Facebook, and is growing at about 200,000 new users per day. Prior to the Facebook launch, iLike had about 3 million members--it doubled that figure through Facebook after just four days. In an interview with Venture Beat, iLike CEO Ali Partovi said the Facebook development is as significant as the development of the PC, Windows and the Web.

But the iLike story also underscores the viral importance of social networks--having a Facebook app puts a developer in front of over 25 million users. Partovi said: "Developers who don't [build an app] are like the people building multimedia CD-ROM software in 1996 who didn't ask themselves if they should be building Web sites."

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