I've not just been spending time looking at social networking services, I've also been digging deeper into wiki. I've still got more to go, but some of these will be of interest to you if you are considering implementing a wiki for your community, or if you are a wiki developer. Zwiki is based on Zope, and thus has a very interesting feature set. One of the more popular features is the topic mappings that it creates.
A blog on social software, collaboration, trust, security, privacy, and internet tools by Christopher Allen.