Read here (cool primer about social software), the social software mechanisms description. The authors takes AIM as an example.
- Identity | Your identity is shown by a screenname, which remains persistent through time. There are incentives not to change this, like having your list of friends stored on the server and only accessible through your screenname. This acts as a pressure to not change identity. Having a persistent identity is more important than having one brought in from the physical world. - Presence | Presence is awareness of sharing the same space, and this is implemented as seeing when your friends are online, or busy. AIM isn't particularly good at group presence and visibility of communication, although other chat systems (such as IRC and early Talkers) use the concept of "rooms" and whispers. - Relationships | AIM lets you add people as buddies. From that moment, their presence is visible on your screen. This is a relationship, you're allowed them to have an effect on your environment. Not terribly nuanced however. - Conversations | Conversations are implemented as synchronous messaging. There's a difference between messaging and conversations. Messaging is just an exchange of text with no obligation, but conversations have their own presence and want to be continued. AIM does this by having a window for a conversation. It's difficult to drift out of it, it hangs there, requesting you continue. Contrast this with email which often is just messaging, and conversations die easily. - Groups | AIM isn't great at groups. Although you can have group chats, the group is transient. People have more loyalty to a group when there's some kind of joining step, when they've made some investment in it. Entering a window just doesn't do that, and there's no property of the group that exists outside the individual user's accounts. - Reputation | Reputation is used more in systems which allow meeting new individuals. AIM's simple version of this is "warning". Any user may "warn" any other user. A users total "warn" level (a figure up to 100) is shown to everyone they communicate with. Unfortunately, it's not a trustworthy reputation system, and reputation is notoriously difficult -- but humans are great at dealing with it themselves, given certain affordances: persistence identities, and being able to discuss those identities with other people. AIM's simplistic relationship system makes reputation not so important though. - Sharing | People like to share. With AIM, sharing is often as simple as giving a friend a link to follow. Other systems, such as Flikr, are about sharing photographs. These act as small transactions that build genuine group feeling.
Another good reference here.