Ben Brooks is making a totally sound point. However for a lot of us, the point where we feel that we can ditch twitter has not yet passed. While it can be argued that we should be thinking forwards, of the future of Twitter and making our stand right now, many of us are not yet at the point where App.net provides a complete alternative. For some of us, Twitter is still important for the reasons it always was.
Because we're slower to move, and reticent to actually cut ties with a valued service, doesn't make us any less supportive of a new platform like App.net, or any less displeased by Twitter's appalling treatment of it's user base.
I am however contributing less to Twitter, and this is a trend I expect to continue; I have primarily been engaging in direct exchanges via @reply than posting what you might consider 'original content' such as a tweet can be considered.