What app.net can do is offer you as a developer the credible promise that whatever you build on it will not be shut down because it conflicts with business decisions made 1, 5 or 10 years from now.

This of course trickles down to the user, since you are virtually guaranteed that there will be new clients for as long as the service exists. With Twitter you are virtually guaranteed that no new client ideas will see the light of day unless they come from Twitter themselves, and those will all be designed to increase your usage of the service with any increased utility merely serving as the hook.