The interesting thing about Twitter compared to e.g. Facebook is that the social graph is very weak. I (and in anecdata, many people I know) use it to follow some "big names", and a splattering of smaller names, for broadcast information. I occasionally send something out, but it's not a strong set of connections. The network effect is thus minimal; migrating over to app.net would be easy, as I'm not that bothered that I have exactly the same followers/followees. When the pain of the service outweighs it's smallish value, I'll just jump ship.
Even more people out there are just passive followers of celebrity names. Facebook or similar could launch a "Flitter" that did everything they wanted, and get a mass migration pretty easily - once people get annoyed enough to start looking around. If Twitter continue on this path, that threshold will be reached much sooner than I originally expected.