Having heard and read this term a few times recently I’ve done a little digging, and come up with the following, which you may or may not find useful.

Working definition of social software by Tom at plasticbag, with some equally good points in the comments.

Are You Ready for Social Software?
“It’s a sprawling, tentacled world, where social dealings are inductive, going from the individual, to a group, to many groups and, finally, to the universe. Or at least the itty-bitty universe of all people using the Internet.”