I sympathise - having escaped the cyber-'friends' scecnario because my girls are all too old for it. Although my eldest grand-daughter who is fifteen is right in the middle of the whole thing, and is glued to her phone and computer.
If you honestly have concerns about your daughter's well-being, given her condition, then i would consider removing her access to the PC for using contact sites. Yes it will make you a villain in her eyes, but peer pressure is never a good reason to go against your parental instincts.
I recall a serious row with my oldest daughter when she was fifteen (history does repeat itself!) about walking home from the local 'teen night' at the night club. I confirmed she was either collected by me, or she didn't go. Cue massive rows - I'm 'old fashioned', 'hate her having fun' etc. ad nauseum, but I stuck out, and she didn't go. The next evening, front page news of the local paper was a fifteen year old girl who had been walking the same route as would have been taken by my daugher, and was dragged into a car, driven around while being raped by the four occupants, and then dumped at the bottom of our cul-de-sac. I was shocked, but so glad i stuck with my decision.
This is an extreme example I know - but better to be thought of as a fuddy-duddy than back down and see your child get into harm's way.
I would insist that she either shares her pass-word for you to check occasionally, or her access is terminated - no compromises.
God, who'd be a parent!!!!!