Ok, I remember seeing a fairly large site for a financial institution attending something like 10 million users. How many servers, you might say? about 40. Ok, add in the mainframe boxen, but still, 40 servers for 10 million users.
According to that measure, Facebook should have 2,500 MILLION users (or 2.5 US Billions), which would make like half the Earth's population. Mind you, I don't think there are that many people hooked up to the Internet, even less likely to be logged on Facebook.
Then again, these "10 thousand servers" might just be 10,000 486 DX2's strapped together! ;)