Wow got so caught up in stats that I forgot to write what i was going to write. Obviously I am not a 15yro, but the problem is trying to look popular by having lots of friends, so they friend everyone that they can. And as other have said, this can leak personal information that shouldn't get out and give strangers an easy way to contact them and location/photos.
With IM previously, you could talk to random people and it really didn't matter. All they had was a username. And as long as you were smart about it, they couldn't find out anything about you unless you told them. Now it is just free for all.