I have become a fan of Ning in the last 5 months. How I stumbled upon it was after attending NECC in Washington, DC this summer. There was an open session of a teacher who uses Ning as a teaching tool. She used it for group projects and other collaboration tools for her students to use for meetings and for presentations. It is a great tool and it has everything in it that you would want for the students to use for collaboration, like blog, email, embed videos, share articles, post forum discussions. I think what is great about it is that NING is based upon common interests. So for example, if I want to share my "expertise" with the world on dog breeding, I can go to Ning, create a network called "dog breeding" and invite others who have the same interest.
Another reason I like Ning is the fact that I can privitize it. This is how I was able to sell the concept to my administration. Since I have a required log in for my ning, I am protecting my "patrons" on the Ning.
The final reason I like Ning is the fact that I have access to past discussions and past information because it is archived. Too many times with a wikispace the information is gone or I have to go to the wayback machine to find it agin.
I think Ning should be something that teachers and librarians should look into for professional development. Since so many of us are in isolation and have a difficult time meeting with others in their field that have great ideas and want to share their great ideas!