Jeff Jarvis has a column in The Guardian – and also on his blog, buzzmachine – in which he argues that the success of Facebook is down to the fact that your (tangible) identity is at the heart of your Facebook identity. He says it better than I would explain it.

He writes:

What is Facebook’s secret sauce? I think it starts with identity. On the otherwise anonymous and pseudonymous internet, this is a place where real identity matters: I use my name and I associate with people whom I actually know… we newcomer adults [unlike school student users] already seem to be developing a rule (borrowed from the similar business site LinkedIn) that we should befriend only those we know; it is an endorsement. So we are the masters of our identities and our communities, which establishes trust. I think internet users have been yearning for such control.

The disconnect between online activity and physical identity goes back to the early days of the internet. The most famous cartoon about the internet is, ‘On the internet no-one knows you’re a dog‘. One of the earliest articles about it to penetrate mainstream media was Julian Dibbell’s “A Rape in Cyberspace“, or How an Evil Clown, a Haitian Trickster Spirit, Two Wizards, and a Cast of Dozens Turned a Database Into a Society, in the Village Voice in 1993, on how people weren’t all they seemed to be in a text-based MOO environment. And these concerns have increased since then, not diminished.
On this reading, Jeff Jarvis is probably right – Facebook represents a counter-trend, and hence (some of) the reason for its success.

One other reason is also mentioned in Jarvis’ article:

At Davos this year, a powerful newspaper publisher beseeched Mark Zuckerberg, the young founder of the hugely successful social network Facebook, for advice on how he could build and own his community. The famously laconic Zuckerberg replied “You can’t.”