My 12yr old can not post a pic without my approval first, and can not be friends with people she doesn't know irl.
Before creating the account we had a long talk about- even though you are beautiful and you have a body to be very proud of (she is a dancer, and puts a lot of time and effort into it) once something is posted it is no longer yours, people can do what they want with it. I also emphasized the fact there were to be no sexualised pics, I know this can be an awkward conversation- but if their using social media it's one they need to understand, as this is not the age to be doing that and as a female there are always better ways to be seen and to get attention.
These are conversation that are always on going, I don't have any issues with DD but I feel like I always need to be on top of it, as some of the pics I see posted by her peers not only petrify me but really sadden me. So I don't think it's a conversation you have and solve the issue. Rather something that links into chats about self esteem, confidence, boys, s*x etc