For my circle of non-tech friends.. Facebook has replaced email and IM. That's pretty big.
And sharing location ("I'm having beer at the pub. Come and join me")
Oh, and they use it for little things like product recommendations ("I need a new washing machine. what do people like?")
And then there's the whole "how do I fix my computer/car/whatever". Lots of people don't Google for it - they just ask on Facebook and wait for the original poster to Google it and reply to them
But yeah.. apart from these things it's pretty useless really.
[1] a slightly-educated guess
[2] and many others as well, obviously
Seems to me facebook is just a bunch of people screaming for attention and collecting "friends" like baseball cards. Useless.