Why? If Facebook finds a monetization engine for its huge userbase, $100B isn't out of the question. They're already making $1B/yr from ads.
That's impressive, but they will have to double their profit several more times to warrant a 100b price tag.
Google has a PE ratio of 20. If you extend that to FB, they'll need to make 5b in revenue to get to 100b. Assuming that last year they banked 20mm, they would have to double their earnings eight times to get to 5b.
I think the difference is in potential. Facebook has more potential than any other company to make huge amounts of cash. What it's making now is (probably) just the beginning.
You can even get more potential from having a lot fewer users, but monetizing each user by a greater amount. Having tons of unmonetized users and looking for a way to monetize them is questionable at best.
However, as I remember, Google had found their monetization engine before their IPO. Fb hasn't and the IPO is supposed to be just around the corner. Plenty of other large sites didn't work out like Google. I mean, Fb could wind-up like Yahoo, lots of users but no great monetization from them. In fact, Fb looks a lot like a better Yahoo since it more or less provides what Yahoo provides in a more integrated format.
Consider, if a brick-and-mortor company somehow offered free storage of real-world goods anywhere in the world, they could easily grow to millions of users too. The monetization question would loom large as well.
You think FB is making 23 billion? I don't.
It does sound a little high, but not as insane as it initially might.
Basically they're going to need to start selling "stuff" for that valuation to make any sense at all.