This is an intriguing thread ... fb clearly provides a useful service for many, good enough that they are prepared to put up with the intrusiveness, the spamming opportunities and security weaknesses.
Joining fb is like going through a trapdoor, which has always inhibited me from joining. The functionality offered isn't really unique, so is it the convenience, the breadth of the userbase, or some pre-existing personal relationship with other fb users that makes it so beguiling?