The user wants to associate, communicate, and interact with other people (such as friends, family, and colleagues) online. (See also the social design patterns wiki for more patterns like these ones.)
These high-level principles have proven to help make social experiences work online.
Just by being involved, engaged, inclusive, a portrait of the self and a person's identity is built within the system which can then be interacted with by others.
When designing your social interfaces, ask yourself what social objects belong in the architecture and how you are going to support them.