Facebook does not face a brain drain. If employees wanted to cash out, they could have sold their shares on the secondary market.
To the contrary, Facebookers love working at Facebook; they enjoy coming into the office of a company that maintains its startup culture while making history.
Indeed, there are few professional rewards as great as the sense that you’re changing the world. When almost one in seven people on the planet use your service, your opportunities are profound and transcend a quick buck.
Put another way, Facebookers are true believers: they know that the best is yet to come, that cashing out now would be like selling Google when it opened at $100 (it’s now at $623). As Paul Ford recently observed in New York magazine, “Even if Facebook never adds another user, it will keep growing: It has become a fundamental substrate, a difficult-to-avoid component of any site or app that requires users to register—making it essential to nearly every major web innovation now and in the future.”
Addendum (5/22/2012): Success!