Social software works bottom-up. People sign up in the system (for example, by downloading an IM client and registering an ID there) and then they affiliate through personal choice and actions (I add you to my buddy list, and you decide to remove me from yours).
Traditional software approaches the relationship of people to groups from a top-down fashion. In the corporate setting, its hard to imagine a person existing without being specifically assigned membership to top-down groups: your team, your division, the budget committee and so on.
Over time, more sophisticated social software will exploit second and third order information from such affiliations
interesting