I imagine most people want the pre-IPO stock. That's the only reason I could honestly conceive. It's the sad truth of the world we live in. Facebook is, like MySpace, always at risk of being overwhelmed by the next social go-to website. Application developers follow the users, not the platform.
Some people may follow the money for pre-IPO stock options, but those are exactly the kind of people you don't want. They are in for the money and will jump ship at the next perceived opportunity to strike it rich.
Most people change jobs because they want a new challenge. They want to build something new, or do something they haven't done before. The fact that stock options are part of the deal is frosting on the cake.
In the case of high profile startups with multi-billion dollar private valuations, the upside potential at IPO may be pretty limited anyway. Stock options are always a gamble. In this environment of limited IPOs they are a big question mark.