Tim O'Reilly has released the results of a fascinating study of Facebook as an application platform. He's got good news and bad news.
The good news has already been widely disseminated: there are nearly 5000 Facebook applications, and the top applications have tens of millions of installs and millions of active users. The bad news, alas, is in our report: 87% of the usage goes to only 84 applications! Only 45 applications have more than 100,000 active users. This is a long tail marketplace with a vengeance — but unfortunately, the economic models (for developers at least, though not for Facebook itself) all rely on getting into the very short head. Here's the distribution of active users among the top 200 developers. (Some developers have multiple popular applications.) As you can see, the drop-off is extremely steep.