Gene Smith has an interesting blog post on the "seven elements of social software":

• Identity - a way of uniquely identifying people in the system
• Presence - a way of knowing who is online, available or otherwise nearby
• Relationships - a way of describing how two users in the system are related (e.g. in Flickr, people can be contacts, friends of family)
• Conversations - a way of talking to other people through the system
• Groups - a way of forming communities of interest
• Reputation - a way of knowing the status of other people in the system (who's a good citizen? who can be trusted?)
• Sharing - a way of sharing things that are meaningful to participants (like photos or videos)

Smith then introduces the "social software honeycomb", a neat and useful visualization of this concept:

The dark green hexagon is the focus of the system; the light green hexagons are the supporting elements.