I believe social media can be a very useful tool for educators to develop and get better in their profession. By following and friending other educators you can access abundance of information about workshops, get ideas, get questions answered, networking opportunities, job opportunities. You can mentor or be mentored and help others. It's a great way to communicate with others that maybe are across the country and will never have the opportunity to meet face to face.
Personal social media and work social media should be separated period. Any page created where you are dealing with professional matters should be just that. Do not follow any friends or acquaintances unless they are in education. Do not post anything personal like vacations or opinions having to do with your family. Keep it related to your job and less problems will follow.
If I were to come across an inappropriate post from a student and it was not posted at school or from a school account having no relation or connection to my school site at all I would still notify my principal because she can contact the parents. I totally agree with Patrick Larkin's approach to the situation. Our kids are still our kids even when they are not in school and i'ts our job to teach them about the dangers of the world with the support of their parents. It takes a village. The goal would not be to get the student in trouble but a "teachable moment" for them to understand the risks and potential consequences of their actions. My suggestion would be after the principal contacted the parents for the school to host some sort of an assembly to prevent and educate other students from doing the same thing.