It woud be useful to hear something from High Fidelity about their views on what social structures might be provided.
I think its a big part of virtual worlds that you can create social networks, friends lists to contact those you meet or regularly collaborate with (and even at this stage test with) and groups that can support contact making, communications and presence indications.
To put in my two cents worth… I think the minimum to get started are
a) friends list to add contacts with in world presence indicators for those in your accepted friends list
b) ability to join each other at a location (already in place so long as the user is online and you know their @name).
c) group creation and membership
d) ability to message another person or a group and it be delivered later if they are offline.