Sad news for App.net :( Unfortunately, I never understood what is/was the point App.net. Somebody did ask that once on Hacker News but the best answer was "it was kinda RSS feed but they now changed focus so they lost me".
Fundamentally, App.Net is a platform that provides a number of services, including Twitter-like one-to-many messaging, private one-to-one and one-to-many messaging (like Twitter DM on steroids), many-to-many messaging (like IRC chatrooms), file hosting, and some less-obvious features like messages generated by and for machine consumption (that are never seen by humans), etc.
These are the nuts and bolts that can be used to build a Twitter clone (which App.Net did and which is the primary "product" that most of their users used), a Dropplr clone, blogging a la Octopress, personal memory logs (like Ohai[1]), Photo story sharing apps like Sunlit[2], etc.
Heck, you could even use this to build a Chess app that lets you play remotely with other players, that automatically records every move ever made, and that lets you access your games and history from any device. Basically, anything that involves sending messages (of up to 2k, plus various bits of metadata) between two or more parties and persisting them could leverage App.Net to power itself.
Of course, that's part of the problem. They positioned themselves as a platform, which meant they didn't market themselves very well to end users.
