If there would actually be a competitive and disruptive service to switch to (besides Twitter), Facebook's early adopters would quit it even faster than they are now. But for now we're just seeing those getting bored with it quit.
Looking forward to see if Diaspora, Altly and Incliq will be those "disruptive" services to compete with it (though I'm not sure if either of them will be disruptive or just incremental). Or perhaps something new will appear soon.