I have bloody hundreds of facebook friends, and you're all quite right that most of them mean jack-all as real friendships. I have maybe 50 or so from my old school, who it's nice to see how they're doing, and maybe message once in a while, but only maybe five or six of my closest school friends who I keep properly in touch with.
Then, about another ten who're still at school, who all added me, presumably for the novelty of having a facebook friend.
Then some from old clubs and societies, a mixture of people who've added me, and those I've added through curiousity or boredom.
Then there are some verrrry random people, such as a couple off TSR who've I've added - again through being vaguely interested to see what they're like in 'real life' (or more real than TSR, anyway)
Then there's the fact that pretty much everyone at my uni has facebook, and people often add each other after only meeting very briefly. I've got to admit, that out of vanity, I'll accept pretty much anyone I've met who adds me. I draw the line at complete strangers though, that's just bloody weird.
Although one of my friends proved that a lot of people don't regard this as a problem - she set up a fake profile, with a hot picture, and took adding people with the poshest names she could find. A huge proportion of the people she added accepted, and this fake girl has 150ish friends!!
So in short, how many facebook friends you have is not a good way to judge character, or anything else about a person really!