Social software is any software that supports group communications. The dynamics of social software are significantly different from traditional interactions.
Social Software and the Politics of Groups, Clay Shirky, First published March 9, 2003 on the "Networks, Economics, and Culture" mailing list (http://www.shirky.com/writings/group_politics).
... a female householder with no husband present, and 17.9% are non-families. 13.4% of all households are made up of individuals and 5.5% have someone living alone who is 65 ...