Since lots of research concerning social software could be derived from MUD research, I read this nice paper: "Hearts, Clubs, Diamonds, Spades: Players Who Suit MUDs" by Richard Bartle

Four approaches to playing MUDs are identified and described. These approaches may arise from the inter-relationship of two dimensions of playing style: action versus interaction, and world-oriented versus player-oriented. An account of the dynamics of player populations is given in terms of these dimensions, with particular attention to how to promote balance or equilibrium. This analysis also offers an explanation for the labelling of MUDs as being either "social" or "gamelike".