Just announced this week, Ning will no longer be free and they will be phasing out this option. Looks like teachers and librarians will need to find a new option. You can read about their rational here. Steve Hargadon, who created Classroom 2.0 Ning blogged about this on his blog about the changes to Ning including a link to a Google doc to post alternative networking options.
I took a close look at the Teacher Librarian Ning that was started by Joyce Valenza (who seems to be in the middle of all things library) Currently there are a lot of resources about the benefits of a well staffed library to student achievement including the video below.
Find more videos like this on TLNing
There are also a wide variety of examples of student work, a discussion boards to exchange ideas and a calendar of events throughout the country.