The whole episode sounds like a copycat of what's happening on the east coast (where it's much more advanced and dangerous). These flash mobs have been reported in the NY Times -
http://www.nytimes.com/2010/03/25/us/25mobs.html
The situation in Philly is bad. Here is an excerpt from their paper on an incident last month:
Business owners yesterday called on Mayor Nutter to stop "flash mobs" on South Street after patrons couldn't shop, dine or get home on Saturday night because of the hordes of teens roaming the neighborhood.
Inspired by Twitter messages to "come to South Street," police say hundreds - business owners say thousands - of young teens stampeded down South Street in waves, jumping on top of cars, knocking over pedestrians and fighting and cursing.
"It was like a tsunami wave," said a store employee.
"The cops were overwhelmed," said a store manager.
The South Street business owners called on Nutter to impose a curfew of 10 p.m. or earlier after frightened managers locked their doors, only allowing customers inside.
Nutter did not return calls last night.
Saturday's was the sixth flash mob to hit the city since last May: three on South Street; two in the Gallery, including one that spread to Macy's; and one along Market Street East that spread to the area near City Hall.
Several store owners and managers documented the stampede with cell phones or store surveillance tape.
A pizza shop owner said that some in the mob were chanting, "Black Boys!" and "Burn the city."
One youth was overheard on his cell phone saying: "Bring baseball bats to South Street."
After enduring months of street construction during a recession, then one snowstorm after another, business owners were delighted that customers had returned to South Street to shop, eat and stroll on the first day of spring.
But the mood turned ominous as more and more teens showed up by 8 p.m. Between 9 and 10 p.m. the packed crowds reached a crescendo, according to police officials who deployed highway patrol, narcotics strike force and other units to assist officers on South Street. A parent, who had seen a text message or a post on Facebook, alerted police about the potential flash mob.
By 9 p.m., Yee Chau, manager of eModa, a clothing store, on South near 3rd, said "It was total mayhem. Kids were out of control. They were wall-to-wall. You couldn't see the sidewalks."
One armed owner, who showed the Daily News his gun permit, protected his business by standing outside with five assistants.
At Supper, a restaurant in the 900 block, bartender Kyle Fennie opened the locked door to let two woman customers out, but a mass of teens descended, and he let the women back inside. During a lull, he walked them to their cars.
