I find a definition about social software. Hope it is useful to understand it. I summary some points which interesting for me. Here is the connection to view whole text.
Social software: software that supports group communications, includes everything from the simple CC: line in email to vast 3D game worlds like EverQuest, and it can be as undirected as a chat room, or as task-oriented as a wiki (a collaborative workspace). Because there are so many patterns of group interaction, social software is a much larger category than things like groupware or online communities — though it includes those things, not all group communication is business-focused or communal. One of the few commonalities in this big category is that social software is unique to the internet in a way that software for broadcast or personal communications are not.
The thing that makes social software behave differently than other communications tools is that groups are entities in their own right. A group of people interacting with one another will exhibit behaviors that cannot be predicted by examining the individuals in isolation, peculiarly social effects like flaming and trolling or concerns about trust and reputation. This means that designing software for group-as-user is a problem that can’t be attacked in the same way as designing a word processor or a graphics tool.
Social interaction creates a tension between the individual and the group. This is true of all social interaction, not just online. Consider, from your own life, that moment where you become bored with a dinner party or other gathering. You lose interest in the event, and then, having decided it is not for you, a remarkable thing happens: you don’t leave. For whatever reason, usually having to do with not wanting to be rude, your dedication to group norms overrides your particular boredom or frustration. This kind of tension between personal goals and group norms arises at some point in most groups.
