It isn't clear exactly what you mean, but I am going to assume you mean using social networking sites such as Facebook as an educational tool. One disadvantage to this is that it removes a certain distance between teacher and students, allowing both access to too much personal information and allowing students and teacher to communicate in ways that might not be appropriate. Some schools have policies which prohibit teachers and students from being friends on Facebook for this very reason. Second, the use of social networking sites in the classroom can be more of a distraction than a tool for education. I have read of teachers who have used Facebook educationally, but I have never read of how these efforts turned out. Social networking is meant to be social, not educational, and when students are on Facebook, it always turns out to be far more social than educational.