Oooh, a pet peeve of mine. Nice.
Kids are like cats: You look, they obey, you don't look, they do what they want.
If you want to protect them, teach them to critically assess situations. Don't make them vulnerable and dependent on you by keeping them naive about reality.
It might not fit into your view of what a childhood should be like or when in childhood things should happen. But when your child wants to do something, it will. If not at home, then at the place of a badly supervised friend. You can only resort to equipping them for the tour they insist on taking.
Help them to grow mature and responsible by granting them the ability to be just that. Help them to trust themselfs and stand their ground by respecting their views, taking them seriously and discussing them like you would with the adult that they feel they already are. Make them resistant to pressure by not using pressure on them, but arguments, and by admitting when they do have a point.
They want to go on facebook? Allow them to and discuss the dangers with them. And please, take their views on fb over those of the moral panic media and politics; your kids live in youth culture, they know what it means to kids to dress up like a whore or a pimp, and you will likely be surprised to learn the truth.