Like FB or not (I don't), it provides a lot of value to a lot of people. While it might not promote the ideals some of us "hacker" types have, on the whole, it is not flat-out evil like, say, some of those toolbar companies or botnet groups.

Additionally, apart from writing in PHP, there must be tons of technical fun and deep problems to solve that makes FB fairly unique.

And also, I'm not really sure how unethical it'd be to work for them, even if you felt they were wrong. Unless you're bringing some rare 4-sigma value to FB, it's likely they'd just get someone else. Might as well be you, where you have a chance to influence things.