God, I hate all this "prepare for an interview" thing. I know, it's nice to know more or less what to expect from it, but the whole "do this, don't do that" makes me sick. Just try not to be too nervous and be honest.
Some Google recruiters go as far as giving you a study guide for the interview, since a good chunk of the interview is essentially a math test, where the math is high level computer science concepts.
Good luck OP! I'm in the same boat, though not with Facebook (The only big company I've talked to is Google). Interviews are starting soon and I've never done any of this type before. Kinda terrified to be honest. My theory is fairly good but I have a tendency to blank out badly under pressure.