But who is keeping app.net from doing the same thing once they killed Twitter and reached a size where they can say: Thanks for all your friends, now get off of our platform.

We need open protocols and a decentralized social network where we have different implementations with focus on different use cases which are still able to communicate with one another.

A good chunk of the protocols and implementations is already there with OAuth, Activity streams, salmon, RSS and such. And there are already open sourced implementations like status.net and others alike.

The solution is already there. We only need to start using it.