Beyond the true fans and those emotionally attached to a product, are the rest of the users locked on any service?
In my mind, beyond Google I don't think there is any social network or service that has a lock on its users, assuming they are exposed to something better.
I think a certain percentage will stick to what they know. But yet if you had a specialized social network for lets say insect collectors - members of the other social networks who loved to collect insects would probably jump ship. And then another question is what would make one "insect collector" social site better then the other? I guess you could offer free insects to users or suchlike. Do something focused and cool and maybe you can "unlock" users...