Codename Max is a cool P2P photo sharing (beta) application from Microsoft. The Channel 9 blog has an interesting video interview with the development team.
Haven't installed it but from the video's description of how it works, the mechanism for inviting friends to share your photos is the standard one - you specify their email addresses (seemingly linked to a .NET Passport?) and they receive an email with a link to click - after which a P2P connection is established between both machines for the transfer of the relevant files.
I saw no mention of support for tracking social relationships, e.g. friends, once invited to access one batch of photos, would be remembered for the next time. Or, even better, for Max to be able to use some common social layer not specific to photo sharing and isolated on the user's PC.
Theoretically possible would be that, when the user indicated they wanted to share photos (or videos, music etc), the Max desktop app would call out using the protocols defined by the Liberty People Service to view the list of friends, family etc with whom sharing was desired. Friends, once invited, could be optionally remembered for subsequent availibility to other social applications.
I take some solace in the fact that physicists who believe in the parallel-universe interpretation of quantum mechanics will argue that this is indeed happening in some other less-divided identity universe.