People are not going to leave Facebook in droves. First they need some place to move too. There has to be an acceptable and viable replacement for Facebook and there is nothing in sight. Twitter won’t work, it has a different structure and won’t appeal to most Facebook users.
That does create an opportunity if someone can roll out something Facebook-like, familier but better than FB. Or something that does the same things as FB but in a new and sexy way that catches the public eye. But that does not happen overnight. Plus it has to scale. Will someone take advantage of the opportunity?