The problem is that Facebook's primary asset is users. They're making money off of them(approximately $1/user in profit and $3.5 in revenue), but it's not clear if they can make exponentially more per user.

They certainly can't expand the user pool by that much, which means that they need to find a way to monetize their existing users more effectively. I think that Facebook can certainly make more/user than they currently are. $5, maybe $10/user is possible. But $100? I don't think so.