Well, technically if you want to use that model, then if each user spends $25 a month on products/services they find on facebook, then after a year(collectively) each user will have at least spent, $15 billion, which I think is worth...well... $15 billion.
True, if TiVo or a cable company had 50 million subscribers, then yeah, it could be worth $15 billion. I don't see a $25/mo product from Facebook, though.
Not to mention that we don't know what percentage of the 50 million are active, participating users.
