This is what I get. Could be totally wrong and if so, please correct me.

App.net allows app developers to use app.net platform for a fee. App developers can integrate their user's app.net logins/profiles/network/photos etc. just like they do on facebook platform. App developers can also create users in the app.net platform. Users never see any ad. They also have good privacy settings. It's like win-win for both end-users and app developers. The only catch being app developers pay a nominal fee.