Let's not forget that Facebook's network is actually a virtual one. The real social graph is independent of any virtualization provider.

Which is why all it will take is a compelling competitor to start the great migration. This is sort of the corollary to the network effect — as soon as there is a better alternative that people really move to, a vast amount of people will move spontaneously.

Imagine if Instagram started right now. Do you think they would accept any offer to be bought by Facebook?