It's been worse in the United Kingdom, with riots in London, Birmingham, Manchester, and elsewhere. Rioters relayed information via BlackBerry Messenger (BBM), the instant-messaging app for BlackBerrys. One text read: "If you're down for making money, we're about to go hard in east London."
Not only were some riots organized via text, tweet, and post, but some mobbers posted surveillance-camera videos of the flash rob afterward on YouTube.
"Face it," says Rock: "Social media are the lamppost of this age."
Philadelphia's flash-mob headaches date from the South Street mobs of March 2010, when hundreds choked the area, leading to injuries and property damage. Last month, 15 to 25 people attacked bystanders at random in Center City, prompting Mayor Nutter's impassioned Aug. 7 speech at Mount Carmel Baptist Church.
The next day, Nutter and District Attorney Seth Williams announced measures to discourage flash mobbing, including moved-up curfews in parts of the city and fines for breaking curfew. The city is working with the FBI to track criminal use of social media.
Everett Gillison, deputy mayor for public safety, says the push involves a positive effort "to reach some of these kids through the very media they use, through Facebook and Twitter, to win the hearts and minds of these kids."
The New York Police Department just started a new task force that trolls Facebook and other services to track down bad guys. Police have arrested one Ten Most Wanted guy, who taunted them on Facebook: "Catch me if you can, I'm in Brooklyn." They could and did.
On Thursday, British Prime Minister David Cameron said he would consider banning rioters from social media: "We are working with the police, the intelligence services, and industry to look at whether it would be right to stop people communicating via these websites and services when we know they are plotting violence, disorder, and criminality."
BlackBerrys are more popular among young people in Europe than in the United States, where smartphones are more prevalent. One reason: BBM is highly encrypted - written and protected in a complex, impenetrable code - and so is nigh impossible to break into or disrupt. Rioters used BBM as a closed network, and Cameron, plainly frustrated, talked of giving police "the technology to trace people on Twitter or BBM or close it down."
Could that happen in the United States, with its First Amendment guarantees of free speech?
Lyle Denniston,e National Constitution Center adviser on constitutional literacy and a writer for SCOTUSblog, which tracks the Supreme Court, says decisions such as Brandenburg v. Ohio of 1969 have set the bar pretty high. "A Twitter or Facebook message," he says, "would have to say, 'We will meet at Broad and Market Street tomorrow at 10:30 and begin an assault on City Hall. Bring your Uzis.' If it's not that explicit or direct, it would be very difficult to argue for regulation."
Besides, Denniston says, the court would favor the role new media played in the Iranian elections of 2009 and 2010, or this year's Arab Spring in Tunisia and Egypt. "They'd weigh the social good of such media very heavily against the government's desire to regulate them."
"We don't want to be known as trampling on civil liberties," Gillison says. "However, you don't have a First Amendment right to use this mass association in a criminal way."
Rosa Golijan, technology blogger for MSNBC.com, says why look at people's private stuff when so much is available publicly? "Many of the kids who have been organizing flash mobs post on Facebook pages anyone can access," she says. "On Twitter, you can see trending topics by city that will show you what people around you are talking about right now."
Pressure is likely to come down on manufacturers to cooperate. Should Research in Motion, maker of BlackBerry Messenger, make its product easier to surveil? Or help law enforcement? If it did, wouldn't its product, whose selling point is privacy, be less attractive to law-abiding consumers?
"As encryption technology advances, it will be a harder and harder question," Rock says. "Manufacturers will have to balance the needs of law enforcement and the needs of their customers."
The summer of the flash rob flashes by. And cities, courts, and police run to catch up with social media, pondering whether there will ever be a way to keep a few bad messengers from ruining it for the rest of us.
About 50 juveniles detained on first night of new curfew. B1.
The fine line of a hate crime. Monica Yant Kinney, B1.
Contact John Timpane at 215-854-4406, firstname.lastname@example.org, or @jtimpane on Twitter.