The question of whether safe social networking sites exist, or at least some safe enough for the school to feel comfortable with, is a good one. The question led me to a post saying yes, SNs do belong in schools, and a link to a project where it seems to be done, The Flat Classroom Project. It's named Flat Classroom because it's goal is to link students internationally, breakdown the walls of the classroom so to speak.
It's based on Ning and Wikispaces.com. I don't get why you need both but if you signup for an account you have to create the wikispaces account as well. Right now they have a special offer - k-12 teachers can get a free $50/hr account for their Plus plan. I lied about my school and don't know how to unlie so I could not signup yet 8-).
You can create blogs, add pictures, text, widgets, and I think you an create forums but didn't see a link for that. Anyone can create an account with a Ning account which opens students to the world - but you can make your Ning a closed one. But then they can't exchange with others outside our walls... so perhaps we can have an adiditonal public account for sharing student work or inviting discussion from students internationally. They could raise crosscultural issues, and issues of native rights etc.
For more... here's the CNET article that got me started
FlatClassroom Project Home: http://flatclassroomproject.ning.com/ more... http://flatclassroomproject.wikispaces.com/
Wikispaces private label tutorials (corporate site): http://blog.wikispaces.com/2010/03/new-private-label-video-tutorials.html ($2000/year unlimited users)
Ning system admin privacy controls http://about.ning.com/safety/ncsafetysettings.php