That's exactly why we're in a bubble. "Because other people are paying even more" is a really bad reason to pay too much for something. But that's the entire basis of Twitter's price--it's people investing in Twitter because other people are investing in Twitter (and other such companies) with no concern for whether that investment will ever return a profit.

So you pulled it out of thin air?

No. Look at the citations others have made within this thread. A rule of thumb is generally anywhere from $50 to $200 per head for social network type applications. See also: Facebook, MySpace, Instagram, etc.