Wired Magazine has an interesting article on social media and rioting:
http://www.wired.com/magazine/2011/12/ff_riots/
The article is written by Bill Wasik, who was one of the first people to come up with the idea of "fllash mobs" and was one of the early innovators in the fad back when it was mainly a prankish/performance type of event. The article is 8 pages and every bit of it is a good read; but the author makes a few points that I think are relevant to Tactics & Training.
1. The author highlights that while the crowd may seem anarchic, the crowd has a sense of shared identity and that if that sense of shared identity includes a low respect for local authority, violence is more likely.
2. The author notes that merely doubling a crowd can more than double the propensity for violence and has an excellent discussion of an event in LA, where a crowd whose "identity" was actually very peacefull and non-violent, turned violent (short version: too many people in too little space, disdain for authority among the crowd increased as the police made impossible requests by ordering people to clear the street when there was no room on the sidewalk for people to move to)
3. The author identifies locations that are more susceptible to this phenomenon than others (shopping malls for example).
The read is detailed and difficult to summarize earily; but I think the article offers a lot of insights that would be useful to TFL members in recognizing when these situations are developing (including videos showing them developing) and better understanding why these situations develop,