Apparently FB hyper-targeted Google employees with ads based on the profile these employees had happily volunteered when creating their FB profile. (Wish I had a reference link).

Also, from what I understand (again, only hearsay, I don't have any proof for this), there were many employees at Google who thought "Don't be evil" was a joke.

And so they decided they will go to Facebook, which was more closely aligned with their morals, as explained in this rhyme -

Move fast and break laws,

Lets show these dumb fricks who's boss,

When shit hits the fan

The CIA will cover our arse