In a groundbreaking move Facebook yesterday formally launched the Facebook Platform at their F8 Event. What is the Facebook Platform? Essentially it allows third-party developers to develop applications that function directly within Facebook itself, not just outside of it as many have done with their existing Facebook API. Build your app, including commercial ones where you keep the revenue, on a platform that's the 6th highest trafficked site in the US and growing by 100K users per day.
For more, you can get lots of coverage via Techmeme including good takes by TechCrunch on the Anti-Myspace, GigaOM on the Social OS, Mashable on F8 Live, SpashCast with an FAQ, and ZDNet on uncorking the social graph. And they're kicking it off with real apps: over 65 developers were showcasing applications at the launch.
This brings a whole new dimension to the meaning of social network platform.