No one is saying that Facebook is worth $15 billion. That number comes because that number (or something close to it, I don't know that they've ever made it official) is sort of what Facebook themselves unrealistically were implying they were worth, and Microsoft sort of verified that by paying $240 million for a small share of their company.
Obviously, $15 billion is utter horse****. Facebook is only worth what they make back in real money, which is nowhere near $15 billion. They are simply valuing the potential (present and future) worth of their company at $15 billion. I'm pretty sure they even know they're not worth that much, but simply are trying to generate hype before an IPO.
That myth is only being further purported by the idiot tech news community, who writes outlandish stories about how Facebook is the new Google and might be worth even more than Google.
You remember classmates.com? And then MySpace completely displaced them? You remember MySpace.com? And then how Facebook completely displaced them? You remember Facebook? And then how....