I couldn't resist the rhyming words.
I think using a Ning for class sounds like a good idea. There are other areas where we try to teach students that work and play are separate areas. There are the registers of language. Facebook is for play, Nings are for work/school.
Nings could be used for discussion groups. There could be a Ning for each period, or put them all together. I have students do an outside reading assignment each grading people. As part of the assignment, students would need to get on the Ning, put a post tell about their book and make comments about a couple of other books.
A Ning would be easier than everyone having their own blog and having to get on each other's blogs. They could just go to one place.
Another teacher at school had a Facebook assignment for her class that the students enjoyed very much. If we did the assignment on a Ning, instead, students wouldn't be tempted to get on their own Facebook account while working... it wouldn't be quite as easy.
I did a search for history nings. Many are from various classes... AP US History, World History... for class projects or assignments.
I found the national Council for Social Studies Community Network Ning. I looked at it, read some of the posts. I found an IB teacher who teaches in Thailand! One of the groups on the Ning is for European History, so I joined the NCSS Ning and the European History group.
So Nings can be used in the classroom for class projects and as a listserve to connect with other teachers, ask questions and learn the latest information in my field.