It's not the norm but it DOES happen. Of course it does. What do you expect when you're bombarding these girls with a society and media that tells them sex sells? That their worth comes from their sexuality and how good looking they are? That sex is 'cool' and if you're not doing it you're some sort of social outcast?
It's a pity but it's absolutely true. Combine that with, say, a strained relationship with a parent or lack of ambition/feeling like there's not much worth working for, and this sort of thing will happen. Too many teenage girls DO do things like take compromising pictures of themselves.
But that doesn't mean that you victim-blame. They're generally naive girls. It's a pity they didn't look after themselves better but the blame totally lies with those who took advantage.
What a great post, spot on!