Pointed out in the comments: Why not use fb.me, which they already owned? fb.me is shorter and easier to remember ("facebook me" being a colloquialism these days, joe.smith@fb.me doesn't sound bad at all). And it subtly encourages people to go back and use the Facebook website instead of sticking with raw email, which I guarantee will be an underpinning to whatever secret email "project" they are finishing up.