As of 2011, there were 1.4 m people in US gangs. That's 40% higher than 2009 (FBI: 2011 National Gang Threat Assessment). Why the rapid growth? Lots of little factors, but the big driver: 17% unemployment (with treble that in areas where gangs are prevalent) and the decaying legitimacy of current power structures. With the ongoing deterioration of the US system, young people are going to increasingly opt to give their primary loyalty to any organization (i.e. gang) that will take care of them.
NOTE: a primary loyalty is any loyalty that takes precedence over patriotic loyalty. It can be family, church/religion, gang, tribe, money, etc.
Something more interesting than simple gang growth/spread, is the increasing presence of gang members in the military (primarily the Army) and the transfer of combat skills gained in Iraq/Afghanistan to the street. The FBI report states that 100 police jurisdictions have reported coming into contact with gang members with recent military experience. This training, gained on the battlegrounds of Iraq and Afghanistan, is extremely good (and for some people, extremely addicting). As we have seen in the recent past, even a single man with combat experience and some weaponry can be a HUGE problem for local police. Here's a handy chart on gangs in each branch of service.
The big worry about gangs in the US military is a repeat of what happened in Russia when the Soviet Union collapsed. When the Soviet Union collapsed economically, hundreds of thousands of Soviet soldiers with fresh combat experience in Afghanistan (and little to offer in terms of skills) were dumped onto the street and into the waiting arms of criminal organizations. This process quickly turned Russian economics into a shooting sport. A place where wealth and firepower became synonymous.
The US, currently running a $1.5 trillion a year deficit with the spectre of HUGE cuts in the military (reduction in force) as an absolute certainty, will dump hundreds of thousands of combat vets onto the street w/o an economy able to absorb them. This is particularly true with the US economy about to start its next contraction w/o even recovering from the last one. Guess what happens next...
Let's get resilient communities going, before the gangs get our kids.