Don't get mad or freak out at your daughter. That will just make her want to be more secretive. The fact that she has pretended that it isn't her says that somewhere in her head, she realizes this will upset you or that it isn't a good idea. BE THE MOM- you have to take control of the situation. Just because we have cell phones and computers, etc. does not make them a RIGHT, they are a privilege.
Sit down and talk with her, OFTEN.Over and over, at breakfast, in the car, before bedtime. Make time for heart to heart talks- they may start out one-sided with you doing all the talking, but eventually she will open up as well. Be objective and not personally critical, but you might start by saying something like,
"I don't know if those things you talk about online are things you are really interested in doing, or if you're just talking. Either way, we are just very concerned because we know that even though YOU would try to be smart and take care of yourself in any situation, a lot of people are just waiting out there to take advantage of girls. Even older girls who are in college have had really bad things happen to them, when all they were doing was trying to have some fun or goof around and we never want anything like that to happen to you. So understand that anything we say or do is coming from our love of you and the fact that we want to protect you. We are disappointed and a little shocked that this is the person you want to show other people online. We know that you're growing up, but this person doesn't look or sound like the smart, talented, confident young woman that we know."
There are a lot of great books on girls' self image and sexuality- find some and highlight some parts and sit down with your daughter to read them. Try to get her to talk about WHY she was doing this? Teen rebellion? Trying to impress her friends or a boy? Or maybe just because the Internet is so anonymous she felt like she could and you wouldn't find out.
Emphasize the dangers. And limit her internet time and even her social life until you feel like she gets it. No matter how resistant she is to talking about these issues or how embarrassed you are, keep at it, over and over, but in a loving and firm way, until you feel you can trust her. If you CAN'T- then be very honest about it. Say
"We've always wanted you to have new experiences and be able to go out with your friends. But the way that you've been using the Internet shows us that you are not ready to handle that freedom, so we're going to have to take it away until you can convince us that you can. We're also going to have to make your curfew earlier and limit your time out with friends. We want to be able to trust you- but this online behavior makes us worry about what you might get into in real life, so you are going to have to earn our trust back and prove to us that you can be responsible before we allow you these privileges again."
Make sure her father is involved also and that you are on the same page with punishments and privileges. Taking away the internet is a definite thing to do- but don't just let her go over to her friends' house or get online through her phone or something to get around you. You have to become the smart tech-savvy parent and know who her friends are and their parents. If she is embarrassed by this, she has brought it on herself.
I'm sure she will be angry, but just hang in there. Someday she will thank you for it.