A bit more generic than that; there's no global setting where user says "this is my preferred App.Net app". When user hits Share in any app (many apps provide such a feature), a menu appears with all available apps user might want to share to. Any app can opt in to be in the menu, based on MIME type of what's being shared.
http://android-developers.blogspot.com/2012/02/share-with-in...