How is it a "decentralised" social network?
Applications with the NoseRub protocol keep information about profile data for each of the contacts. These profiles get synchronised between the applications/social networks ... and can be used by any other NoseRub service.
NoseRub uses already available standards like OpenID, RSS and FOAF to provide the goal of having a truely decentralised social network.
Decentralised means, that people may be able to store their social network on their own server and those servers to interact and synchronize automatically. One company doesn't own your data and social network anymore, you own and control it.