Facebook is picky about which app is allowed to use your data. Twitter, for example, is not allowed to use it anymore. So when you develop an app, just relying

Facebook is picky about which app is allowed to use your data. Twitter, for example, is not allowed to use it anymore. So when you develop an app, just relying on facebook could lead to a situation where you suddenly don't have access to your friends anymore (from within that app). Most apps also have their own user databases and just use facebook data for import. With app.net that's the one point where your user data would be saved.