Flashmob in Barcelona Photo: Eurovision TV  
Now, there are two definitions of a flash mob:

1. a group of people mobilized by social media to meet in a public place for the purpose of doing an unusual or entertaining activity of short duration. Here's an example from Ottawa, Canada, 2010, also embedded below. Indeed, that is probably how the idea got started. But the better known usage, alas, is:

a group of teenagers who have contacted each other by cell phone and social media and gather in an area to trash it or to mug and beat passersby.
For example, from 2013, “Hundreds of teens trash mall in wild flash mob”:

“They were playing the ‘knockout’ game,” said Shante, a 21-year-old perfume merchant, in reference to a violent trend in which teens try to knock out an unsuspecting victim with a single punch.
and “Teen Accused of Inciting ‘Flash Mob’ Violence” (also 2013):

The 16-year-old boy's arrest comes after authorities file charges against nine other teens in connection with a wave of violence July 16 along the Hollywood Walk of Fame Here is an example, also embedded below.
Social media enable a large group of people to co-ordinate a criminal event. Imagine trying to do it with an old-fashioned rotary dial landline telephone, reaching only one person at a time (assuming that person is even available to answer). Obviously things just could not happen the same way.
As one analyst puts it, in connection with the recent controversy over a police officer roughhousing a teenager at a pool party in McKinney, Texas, social media played a role:

The more obvious “larger truth”–but one no one is talking about–is that police still have no idea how to handle the “flash mob” phenomenon–the sudden appearance of large groups of teenagers, coordinated through social media, and often behaving badly.
The crowd in McKinney was rowdy but not seriously criminal, hence the controversy. One possible help would be for the police themselves to make better use of social media. It doesn’t help if teens—rowdy, criminal, or otherwise—are way ahead of them. Entertainment flash mob:

Flash mob on the edge of violence:

Violent flash mob:

Denyse O’Leary is a Canadian journalist, author, and blogger.