People spend a lot of time on paying attention to social networks, but I don't think enough time is spent on social features. A social feature is one that brings another person into your digital experience... whether its "send to a friend" or commenting or subscribing to a friend's photos. It actually needs to be a feature, though, and not just "add friends" w/o any context of why you would be adding these friends.
Social has become a bit of a Web 2.0 buzzword, so I was thinking today about some of the concrete value propositions that building in social features represents.