Which assumes people don't move to Facebook email.

Migration of my friends to FB email seems far more likely to me than migration of all my social graph to Google Buzz.

It would also be trivial for FB to allow response to notifications via external email if they decide to do it.

I think people are seriously underestimating the value of Facebook's existing graphs. They've been shaped over a couple of years now to what most people want them to be. The vast majority are not going to leave that behind for a me-too service.