Is there anyone you know who is close to her that you would feel more comfortable talking to? If people continue to ignore the problem, she could die from this. It's a troubling reality. We were posed a similar problem last Saturday night-at my college campus there was someone way too drunk. People around me were hesitant to call security, but for me it was a no-brainer. I was informed by hospital officials that if that call wouldn't have been made, he would have died from severe alcohol poisoning. I don't know how I would have lived with myself if I had seen this problem and not done anything about it.
Lesson: it is something that should be addressed if you're concerned. I wouldn't call her out on facebook itself because she will probably get very defensive and close herself off to you. I have studied a lot on anorexia-because it's always been something that has struck me. Like I said, your best option would be talk to someone who is currently close to her and see what's up. If that's not an option, I would encourage contacting her parents if possible...it would be best to set up a face-to-face meeting with them. If that won't work, I can help you brainstorm alternative plans. I don't know about you, but once I see something terminally wrong, I feel it's less of a right than a responsibility to do what I can. The book Wintergirls shows what can happen when girls who need help don't get it. It's really really sad.