A private social network
Ning is a nitch product, which is aiming at private collaboration. While the teacher, or creator of the Ning site, has to pay a monthly fee (starting at $25), the participation of the students is free. This allows for protecting the privacy of the content and the privacy of the students. To some teachers this may be worth the $300 annual fee, especially if they do not have a school provided Learning Management System. Most LMS vendors today support similar features to Ning.
I also found that closing the Ning account is not straight forward. There is no way to close the account online. I submitted a help desk ticket as instructor, which generated a response, that I need to email the help desk from the email listed on the Ning account. Once I did this I received no further confirmation. To open the account I was asked to establish a Paypal subscription. I was not able to cancel it through Paypal. The account didn't get closed until I called the phone number at Ning customer service.
How I Use It
I created a welcome page and invited users to create personal blogs and discussions about the course subject matter. Students did not want to create a new account on yet another social network just for the class. It seems that similar features are available in our school LMS. Specifically blogs, discussion forums, journals. The accounts in LMS are automatically created for students.
From my perspective, since I can grade student discussion forums and student blogs in the school LMS, it provided a greater value than Ning.