Riots have been part of America since before it's founding. Sam Adams and other patriots organized some pretty violent riots.
Political riots after the revolution were very fierce. In one such riot a former revolutionary war general was one of the people dragged out and lynched because they supported the wrong party.
As time passed the nature of riots started to change, to a degree. Police riots became more normal. Most if not all labor riots from the 1840s on were police riots.
What was unusual about the race riots of the 60s is that the actors were African Americans. Historically a race riot was whites rioting against blacks,
Flash mobs such as in the article were predicted in science fiction. Larry Niven in one of his short stories described them. Although his story involved novel forms of transportation rather than communication.
I kind of doubt the hipsters tweeting at each other are going to emulate the Watts riots. More likely they'll throw garbage cans through the windows of Starbucks like the trust fund anarchists did at the WTO demonstrations.
Of course throwing garbage cans gave cover to a pretty large police riot in Seattle. So who knows, maybe we should start arresting those kids breaking into song at the food court.