That's just simply valuing each user by dividing Facebook's estimated valuation by the number of users. I'm sure Facebook will not only continue to improve their revenue to constantly please the pencil pushers, but is it so crazy to value each user at $125? Remember, that's not revenue; it's just value per user.

Most public companies trade for at least 12 times their earnings. Google is 20x earnings and their market cap is $190 billion. Your math would be assuming that Facebook will have $0 in assets and will be trading with a P/E of 1. Earnings of $6.25 per year, per user (at 800 million users) justifies a $100B valuation at a 20x P/E with $0 in assets and no expected future growth.