I remember this argument back in '01. "Google is amazing but how will it ever make money?"
And they were right. Then Google came out with AdSense/AdWords, which accounts for almost all their money. Unless Facebook plans to sell data for cash, their best hope is to come out with their own ad program to rival Google's.
I guess if they started to charge $10-$100 per fan page per month they'd clean up a bit and would make some nice cash. Is it innovative though?
Facebook's following isn't loyal enough to pay monthly fees, especially on a model that needs your friends and everyone else on it to keep you coming back for more. A fee would destroy Facebook.