The hiring process was a long one, but surprisingly pleasant. I met with some folks early on with ideas of how to use their current data with a few interesting twists. It peaked their interest and many 'informational' interviews later I had two official screening phone calls and then four in person interviews. I taught myself the basics of MySQL before one interview thinking that this was going to be a prerequisite. That turned out to be a good move. My advice would be to approach the interview considering real-world problems, a thoughtful launch plan with concrete data to support it as well as risk and mitigation strategies in case it needs fine-tuning from user feedback. Be quantitative but also imaginative. Facebook has some amazing people, so be prepared to go big, or go home.