Distributed social networks are collections of people hosted on a collection of servers interconnected by the internet. Each distributed social network appears to be formed by people using the same application. They have functions similar to Google + and Facebook, except that they allow for any person to host their own server and have it interface with a larger social network. Perhaps this larger social network could encompass people beyond those using the specific application. The inter-connectivity of applications seems to reflect or parallel a move towards open standards to build a federated social web.
According to Wikipedia's description of a distributed social network, there appears to be an exception. That is, plug-ins.