It's also a huge difference in scale. MySpace ultimately stumbled on a comparatively small stage. IIRC at it's height they had maybe 80-100 million legitimate accounts. A mass migration of say 20 million users in a short period of time is something totally different than say 100 million users. FaceBook also had about 50% as many users by the time MySpace hit their numerical peak. So to apply that to Twitter I think some new service would have to grow about 100 million users and then steal about 100 million from Twitter.