Not saying that 15Billion isn't ridiculous, but the formula is n't nearly as simple as
value_per_user = valuation / num_users
value_per_user = valuation / num_users
an easy example: if facebook made 15B per year with 5B in profit, they could distribute dividends of the 5B each year to stockholders. In only 3 years, your original investment would be paid back and it would be all profit (to the investor) from that point on.
Would you think that was a good deal? I would! So good that it'd get bid up on the open market way past 15B.