I've often thought of deactivating but at the moment its still the most convenient way of keeping in touch with friends from various stages in my life. This is mainly because most, if not all, of my good friends live miles away so catching up down the pub isn't really an option. Facebook to me is like a virtual pub I suppose, as others have described it.
My wife and I have a running competition to see who can have the fewest friends on facebook. I've got it steady under 50, probably only 6 or 7 regulars who I keep in touch with and we share interesting stuff. I don't think I can really maintain a meaningful friendship with any more people than that but there are maybe 10-15 peripheral people who I'll check in with every now and then just to see what's going on. The rest are people who I might possibly contact at some point so I just leave them on there as friends. I could remove these people and I understand that deleting someone from facebook doesn't delete them from real life too (they still exist and are contactable!) but I just leave them on sort of like an address book rather than a collection of current and active friends