This is my reaction to New York City imposes new social media rules for teachers. Click to read the article, it's worth it.
I agree a set of rules should be established. Every day I see people of all ages posting things that they really shouldn't. That stuff doesn't go away, particularly if it's embarrassing.
As for the schools mentioned in the article who ban social networking outright? I'm waiting for them to ban computers and start teaching how to bang rocks together. When it comes to technology, you can't TEACH it if you can't USE it.
Social networking is one of the things that permeates the internet right now. If I had to ban all social networking tools from my classroom over half my curriculum (including Edmodo, which is a spectacular teaching tool) would be toast. I'd be back to teaching just MS Word, PowerPoint, and Excel for 180 school days.