We know very little about how much money Facebook actually makes at the moment - much like Twitter. My guess is that both companies are already very profitable but playing their cards very close to the chest. They have nothing to gain from letting out how much money they're making (the hype is already stratospheric on both), and they have a lot to lose (copycats, spammers, etc). All those who know the real numbers are under NDAs so strict that if they so much as farted an information leak they'd spend the next 5 years in court.
You can fool some people some time, but you can't fool a whole battalion of dot-com-bust-battle-hardened top notch VCs and angel investors out of hundreds of millions of dollars for 5 years in a row. The private investors know what they're investing in, and it's probably not hot air.
How? (I tend to agree with you, but when I ask myself "How?", I don't have clear answers.)