I like what I see on Ning and Classroom 2.o
It seems to me that these are the Web 2.o version of the 1990 internet users-groups and list serves that I was involved in.
The college level physics community set up physnet in 1989 and physl in 1990 so that physics teacher around the country cold collaborate on teaching ideas. This was pre WWW days, no graphic interface. Every thing was text driven and there were not user friendly. In addition there was no spell checkers. It was all about communication.
This site takes this concepts to the next level. It adds a cooler interface and all of the social interaction tools such as grouping tagging in near context commenting. This is a great improvement over the users-groups and list serves that were a challenge to follow a discussion thread and did not have the capability of images. It was real challenging to discuss physics with no pictures.
Physnet has come a long way since 1990 but physl (an e-mail list serve) had died. Physnet is not fully Web 2.0 but is making progress there.
I can see a teacher setting up a social network site for their class so that students have a place to discuss the class and get input from the teacher on the learning.