There was a time earlier this decade when flash mobs--the act of people suddenly gathering in a public place to do something unusual--were harmless and often humorous. In 2008, for example, people in 35 cities engaged in what may very well have been the world's biggest pillow fight. But as the AP reports today, with the London riots as backdrop, sites like Twitter and Facebook are both making flash mobs more popular and exposing the ugly side of the practice. Criminals, the AP explains, are exploiting "the anonymity of crowds, using social networking to coordinate everything from robberies to fights to general chaos." The article, which is worth reading in full, details how the flash mob menace is manifesting itself in the U.S. and around the world (the picture above shows police arresting an ecology activist during a flash mob in support of a forest in Moscow).

The AP's report coincides with Philadelphia Mayor Michael Nutter's announcement late yesterday that he would combat the flash mobs plaguing the city by instituting a temporary 9 pm curfew for anyone under 18 on Friday and Saturday nights in two key areas of the city. "If you don't even bother to pick up your child, we are immediately calling the Department of Human Services," Nutter warned, "and you can then be taken to court for other violations including neglect of your child" (violations will also result in fines). Philadelphia's mob violence--which has been an issue for a couple years now, has included an assault of several people on the city's popular South Street strip and the looting of a a Sears store in a Philly suburb.

And if you're curious about that global pillow fight, here's a picture of how it played out in Brazil:

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Uri Friedman