Flash-mobs: remember those fun, carefree gatherings of people, made possible by social media, who pretent to be Best Buy employees, or sing, or dance, or walk around like zombies? Well, the word has picked up another use: to describe sudden gatherings of people,also enabled by social media, who gather for the purpose of perpetrating crimes.This new kind of flash mob is very dangerous. An attack this last week, in Memphis, left several people injured, though it easily could have been much worse. (Remember the story of Reginald Denny? Once a mob has you your life is entirely in their hands.)

Here's a thought:

We've seen large protests, enabled by social media, topple governments. (i.e. the Arab Spring)
We've seen flash-mobs (social media enabled) loot, pillage, and brutalize people. That's what happened in Memphis.
In the USA, we haven't yet seen a social media enabled riot. We saw hints of this up in Ferguson a few weeks ago, but nothing on the scale of the LA race riots. The thin blue line proved thick enough (mostly) to contain it.

Here's an example of what this might look like:

I expect, when this first happens in the USA, it's going to be very bad. Once it's done succesfully once it's probably going to spread.

For the "protesters" this is reasonably safe, probably good at building camaraderie, almost impossible to prevent or prosecute, and creates a huge media splash. Very epic looking. It's also really cheap. A molotov uses about 8 ounces of gas, a used glass bottle, and some rags.

The rioters will completely outmaneuver the cops - they will be so far inside Law Enforcement's OODA loop that all the cops will be able to hope to do is document the crime scenes before they are too badly spoiled.
I expect that one rioter will complain to his group of friends about some slight/injury - the inflicter will then be the target of 20-100 attackers, who gather nearby and then descend in a matter of seconds. Multiply this by 100 events over the course of an evening - it would only take 2,000 - 10,000 people.
Skillful employ of powerful weapons will be all that will save the intended victims.

Two pieces of good news:
1. These kinds of attacks are probably only possible where there's a high population density.
2. In most parts of the US it's possible to legally own the kinds of weapons needed to stop large numbers of attackers - specifically "high cap" magazines and semi autos.