Yeah, I signed up for Texas School Librarian Ning a good while ago, but just keep forgetting to check in and add content! Of course, it was TX school librarian extraordinaire Shonda Brisco (soon to OSU as professor!) who started this Ning. And other heavy-hitters in our realm, like Naomi Bates of Northeast, Carol Simpson from UNT-SLIS, and VWB and Dr BB of Spring Branch are on this Ning, too.
A ning can be of much more manageable size than the biggies, but you do have to get to 'critical mass' - either of members with a little to say or a few with quality content to contribute - before a ning would be truly useful.
Perhaps a ning could be used in school for teachers to communicate with one another in group way instead of interminable rounds of e-mail to answer a convoluted question. Hopefully our district would be okay with unblocking a given, work-related ning, even though other blogging and social networks are off-limits from school computers.