If you are considering online content management, Jason Mayfield has a very good posting titled 50 Content Management Systems on his blog Open Jason. Some of the applications / software on this list are well known but others are not. All in all, it is a good list, and even though this posting is 6 months old (I only stumbled across it today) it is still worth checking out.
There is also a shorter and more recent posting by Dion Hinchcliffe on Zdnet titled: Ten leading platforms for creating online communities.
In addition to the list of social networking platforms, Dion also has some very sensible things to say about creating social networks. After all it takes a lot more than a platform to create a community. If you have not already looked at Dion's post, check it out. It may save you a lot of time and heart ache when designing and supporting your own online library communities.