Seeking to keep pace with changing technology, Illinois toughened penalties Saturday for those who use social media and text messaging to organize violent "flash mobs" like those that have occurred on Chicago's Michigan Avenue and in other tourist areas.

Gov. Pat Quinn signed legislation doubling the maximum prison term for offenders to six years. The legislation was in direct response to recent incidents in downtown Chicago that left business owners fearful that tourists and other visitors would be scared off.
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Police say groups of young people used Facebook and Twitter, as well as text messaging, to organize and publicize a mob action along Michigan Avenue in March. They say several hundred people — most of them teenagers — ran up and down the upscale shopping area, yelling and bumping into people.

http://www.foxnews.com/politics/2013/05/...dia-flash-mobs/

These gangs are responsible for closing one of the busiest malls in Charlotte, N.C. Eastland Mall was a great place to shop but gradually these gangs were hanging around outside, then inside, making it impossible for shoppers to go there.
After a couple of years, the retailers pulled out and the huge building sits empty. I expect to see more of this in the future unless there's a serious clamp down on this kind of activity.