Clay Shirky’s talk at ETCON, blogged by Cory Doctorow, was about social structure is collaborative software. He gives a cool example of group dynamics:

You were at a party and you got bored. They you don’t leave. Why don’t you leave? But 20 min later, someone gets his coat and everyone leaves. Everyone else was bored too, but the triggering event let the air out of the group.

This is called “the paradox of groups.” There are no groups w/o members, but there are also no members w/o groups.