When a massive flash mob rampaged outside the Wisconsin State Fair injuring 11 fairgoers on Aug. 4, old-fashioned police work quickly led to the arrests of 31 people.

When 25 teens looted a 7-Eleven on Aug. 13, outside Germantown, Md., local police posted a surveillance video on YouTube and visited a local high school with pictures of the perpetrators. Within days, 15 of the 26 suspects were identified.

And just this week, two teens were found guilty of orchestrating a "flash mob" style beating in Philadelphia that left one man with a broken jaw.

"Downtown is not terror town," Judge Kevin Dougherty admonished. "Philadelphia will not be a laughingstock because of a few individuals who decide to hunt human beings and laugh about it."

Judge Dougherty is not alone in his outrage as cities across the US struggle with how to prevent and prosecute a new spate of violence organized over social media.

And while the ability to track perpetrators and even potential lawbreakers on Twitter and other social media platforms offers a powerful, futuristic vision for policing, the real-life police reaction to the "flash mob" phenomenon has so far been more Sherlock Holmes and less Blade Runner.

While New York City just established the country's first Social Media Unit, and the Los Angeles Police Department have hunted down criminals using Twitter hashtags as digital fingerprints, only 30 percent of US departments have an active social media policy according to International Association of Chiefs of Police's Center for Social Media study. "We don't have anyone who has social-media expertise," said Janelle Smith, a spokeswoman for Germantown, Md., police.

Indeed, the most common police strategy so far for preventing flash mobs has been the tightening of curfews and a boosted police presence, all of which have been at least somewhat effective in Milwaukee, Chicago, and Philadelphia. In Philadelphia 50 teens were arrested for curfew violations two weeks ago as Mayor Michael Nutter established a 9 p.m. weekend curfew.

In some ways, traditional law enforcement strategies are the best police can do, some experts say. "You cannot stop flash mob activity," Michelle Ferrier, an associate professor of communications at Elon University in North Carolina, told the website All Headline News. "It is a tactic of a frustrated citizenry that uses lax vigilance and group identity to hide individual criminal activity. The only response is greater vigilance and stronger penalties for those caught."

Outside the US, police have had even more success making arrests in the aftermath of urban riots. In Vancouver, police set up a digital "wanted poster" website after the Stanley Cup riots in June, allowing citizens to look at pictures and put names to those they recognized. Police arrested 101 rioters in the aftermath, partly with the help of the website.

In London, the city's massive network of CCTV cameras provided identification pictures for police, who caught more than 1,000 people after the recent riots mostly the old-fashioned way: on foot, asking people if they knew the people in the pictures. "It was really technology augmenting old-fashioned police work," says Lauri Stevens, founder of ConnectedCOPS, a blog about social media-enabled policing.

And though the power of social media policing — facial recognition software, for example, and Twitter apps that can clue detectives into broader criminal networks — seems formidable, there's another catch: Casting wide nets across cyberspace could tread on civil liberties and have a chilling effect on free speech.

"Social media creates a web of people involved in criminal activity and who might know those people, and to be able to see those networks is a pretty amazing thing from a policing perspective," says Kristene Unsworth, an information policy expert at Drexel University, in Philadelphia. "But that's also an incredibly wide, wide net to be searching for wrongdoing. That means we have to be really careful that we're vigilant about questioning these [police] activities, to make sure the checks and balances are rigorous enough that they can deal with this kind of technology."

"A lot of police officers are really concerned about using these technologies," Ms. Unsworth adds. "They don't yet know legally where things stand."

Those concerns are also being debated outside the US. Germany, for example, has banned police from using social media-based facial recognition software in fear it will trample on individual privacy rights. And in Vancouver, police attempts to match riot pictures – including of those who were not caught in the act of doing anything illegal – with the state's driver license bureau's database was cancelled after privacy advocates raised concerns.

In the US, where free speech is enshrined in the Constitution, authorities’ attempts to curtail mobilization via social media are also making news. The decision by Bay Area Rapid Transit to temporarily shut down four cell phone transponders on Aug. 11 to scuttle a planned protest against BART police drew widespread criticism from free speech advocates.

On Aug. 5, Cleveland Mayor Frank Jackson vetoed a city council ordinance that would have criminalized the use of social media to incite public disturbances. He felt it was too restrictive of rights to assembly and free speech. "Use of this technology in a criminal way and how we react to it – without throwing away the Constitution – is a challenge we all have," Jackson says.

To help combat "flash mobs" and other crime enabled by social media, a conference called "Social Media, the Internet and Law Enforcement" (SMILE) will convene in Dallas in September.

"With the flash mobs, police are struggling with the rapidity — it's just so fast, and it's new territory for them," says Ms. Stevens of ConnectedCOPS. "They'll catch up, though. I have a great deal of confidence that law enforcement is not going to be outdone."