There seems to have been a lot of talk recently about using Open Source software for social networking, which has been something that has been at the back of my mind for quite a while now. If you could take a web site and using the presence information and IM features from jabber, you already have the beginnings of a social networking or community site.
The main advantage I can see is that anyone could start their own social networking site, rather than joining one of the larger existing (closed source) sites or basing a community around a web forum (which I tend to avoid if at all possible).
Now if I can just properly gather my thoughts on this I would start some coding.