Facebook friends are actually more like Facebook contacts. People will most of the time just add people they for example just saw walking in the hallway. The main reason for this is because it's almost like you go up in facebook popularity by having a lot of friends. In fact, a facebook user could have 600 friends and only 10 of them being real friends.
Person 1: "Wow I think he's one of the most popular kids in our school."
Person 2: "Why the hell would you think that?"
Person 1: "Because he has like 900 friends on facebook!"
Person 2: "Dude, that kid gets made fun of everyday, he has no real friends. Just Facebook 'friends'."