The question isn't really "Can social networks be good for students?" Discussing school subjects via the internet can be a valuable tool for understanding the subject matter. The question is "Is Facebook the best platform for such social interaction?" Having set up & moderated numerous discussion forums (20+, at least 4 different types), I would say that Facebook is NOT the best tool for the job. The ONLY advantage Facebook has over some of these other platforms is that so many people are already on it and therefore it is very easy to get people involved.
The future of education is going to involve an increasing reliance on technology. Social network use is one of many tools for the student, and which will likely be more important into the future.