"Turns out App.net currently has approximately 20,000 users, of which a small minority seemingly dominates the conversation: Basch estimates that 250 users (1.25 percent) have so far accounted for half the posts." http://thenextweb.com/socialmedia/2012/09/07/off-slow-start-...
So once every 3-4 days a new active user is born..