A number of folks, including me, have wondered why the term "social software" has picked up traction, when there's a whole field of academic computing (computer supported collaborativ work (CSCW)) which encompasses just about every aspect attributed to the term.
Adina Levine is working on drawing out the interesting new features of social software. I'm still puzzling out if there's really something there, but elements ring true. For a half a minute, I'd bought into loose coupling but realized that many of the services cited (Technorati, Flickr) are even more centralized than USENET ever was.
Maybe what it boils down to is that these systems embrace the two aspects highlighted by Clay Shirky: first class group formation, and sociable media (my term). Anything else is just reaching.
Hat tip to Seb Paquet