Compare how you would reply to an email notification from Facebook or Twitter: you'd have to login to the site and post your reply (or even see the actual reply) there. With Buzz, it's inline.
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.
For me, I use several of Google's products daily, and always have my Gmail open. That's why I like Buzz, because it seems like it fits me well.
What do you see happening to twitter then, if people move to FB mail, in addition to the introduction of Buzz?
You can actually already do this by replying to the notification email!
A lot of this was elegantly* evolutionary, but it's nice to see them put a name and publicity push behind it.
*In the non-mathematical sense.