To be very honest, she may not want you to know. She'll tell you on her own when she's ready. I use to do the same thing, sratch myself. Just try asking her about anything that's bothering her, remind her you will always be there for her. It's hard to admit to anyone that you're hurting yourself. I didn't tell my friends until I was in too deep. If you're really worried tell an adult you trust that you suspect something. I didn't tell anyone until I was "addicted" to it. But if you really want to confront her I would suggest:

"Hey _____, I didn't mean to see anything but when I logged onto your Facebook to post something, you're chat popped up and I couldn't help but notice the conversation..." Complete with your own words. 

I hope this helped.. :)