Following on from my previous post about identifying social networks that are in your niche and which you can join and hopefully gain some more targetted traffic.
Ning has hundreds of thousands of networks of all sizes that have been created by people themselves, using the Ning template and support. I have identified several networks that are appropriate to my blogs there and joined them.
You just need to go to the Ning home page and start searching for popular networks. Unfortunately, your task is not simplified by them being categorised into groups, so it's a bit of a haul going through them - but worth it.