After reading your initial post, I was really glad to see such a great response from so many people. As a teacher of middle schoolers and an avid gamer myself, it is very unfortunate that so many young kids are being exposed to such material so early in life. However with easy access to the internet and social media via phones, computers, tablets, gaming consoles, and the like it falls on us as adults to be much more proactive in our approach with our kids gaming habits. I am all for your daughter playing games online, but I will say in the same breath that it is your responsibility to protect her as best as possible. The tools mentioned in this thread such as parental controls, limiting voice and text communications to friends only, and you controlling who she is able to "friend" are probably the best options. Regardless of the path you choose, it is going to require active monitoring on your part.
Good luck and thank you for caring what your kid is doing online, it makes this teacher and parent happy.