Hmmm. If we look at some past examples of cases in the consumer internet space where a new entrant overtook an entrenched player (e.g. Google vs. other search engines and Facebook vs. Myspace), I think you need to have something new and compelling about your application to drive adoption. I don't know of any cases where just cloning an existing app was successful for a startup. Has movement to a new player every been driven by openness concerns?

What can you do that is different from Twitter and cannot easily be copied by them?