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.
Not to mention that we don't know what percentage of the 50 million are active, participating users.
That's a bargain compared to GeoCities, which, I believe was purchased by Yahoo for $1100 per user.
Of course 100x is an extremely high valuation multiple for a company based on fundamentals instead of speculation, so if they can't monetize, or if they stop growing, or if attrition goes up, then their multiple goes down and then they might have to make $6, $12, or $24 in profit/user/yr, which I'm pretty sure is unreasonable.
Moral of the story: we're all guessing. If they can monetize, then every doubter is wrong. If they can't, then every doubter says "I told you so."