EDIT: I don't see how the above statement beckons downvotes. Yes. It was an assumption, but not absurd one at that. Here's one such article that talks about fake accounts -- http://www.cnn.com/2012/08/02/tech/social-media/facebook-fak...
EDIT2: Last year Facebook said 4.8% of their MAU was duplicate, misclassified, or spam accounts. That's 51+ million today assuming the percentage has remained constant.