A person who uses facebook to update not only their facebook status but their twitter status also.
A 'Facebook Twit' is obsessed with posting the same updates on both networking sights about his/her mondane activities that noone really cares about.
A: Have you seen what Ben is doing tonight?
B: Yea. 'Ben Smith is taking a bath'.
A: Haha. Yep. Did you see it on facebook?
B: No, I saw it on twitter.
A: Ha! he is such a facebook twit!