I think I just found a story that shows that Shirky is right. Social media does make it possible not only to organize but to mobilize groups of people. Unfortunately it is for nefarious purposes, but it is still true. Flash mobs are being used to rob people. Kids (and I doubt it is only kids) are getting together via twitter and other social networking sites to rob stores. They flood a store, rob it and then disappear into the crowd before the police show up. Thieves then use the same social networking sites to offload their ill gotten gains. It is pretty ingenious actually, and it certainly proves that groups can be organized and such via Twitter!
Take that Gladwell!