Things are changing very fast in this industry. So even 10 years and 2 billion users are optimistic numbers. And if you take 2 billion users, most of these users are from very poor countries, it is extremely hard to monetize these users (almost orders of magnitudes harder than users from rich countries.). That said, I don't think that Facebook is overvalued by orders of magnitudes, and the 37signals article did not make much sense to me.