I think the ‘social network’ aspect of these ‘flash mobs’ has been overblown. I doubt they would find a list of 50-100 folks that received a text message from an organizer. I could be wrong, but I just find it hard to believe that it is this organized. My guess would be more like a small group gathering at a traditional hang-out (park, bball courts) and then that group heading over. Maybe a couple texts out of the group to let some friends know what is going on - 1 text to a park a few blocks away that is also a hangout and you have a small mob with just a couple of text messages.
You would be wrong. My guess is you do not hang out social sites. It is eyeopening.