Student Spin off

Hello, I am trying to trouble shoot potential issues as much as I can before I begin my first ning site with students. I am interested to know (and I can't seem to find information if it exists) what the potential is for students to go and create their own nings once they are a member of the environment? Which isn't bad in and of itself. However, since I introduced them to this social network, if the hypothetical site then begins to show material of questionable substance, doesn't it then reflect back on me? In short--how much freedom would students have once they were a member of my classroom network? Any wisdom, facts, or experience would be quite helpful.