Depends on how you use it.
If you're an eejit you'll have 500 friends cluttering up your news feed and it'll be full of spam, drama and nothing of much import.
I have under 40 friends, and I use it to keep in touch with people I knew at university who I don't see anymore, people back home in Norwich who I only see every four or five months, and organise and arrange my gigs in Belfast, as well as letting people in the city know where and when they are.
Use Facebook responsibly and it can be a really useful tool, it's mostly people who let it get out of hand who end upnot enjoying.