Easy. You get the Federal Govenment to monitor (i.e. read) every single email, tweet, and facebook posting that travels on the internet, as well as every cell phone conversation and text message that travels the airwaves.
Short of that, no there is no way to 'monitor' flash mobs. A flash mob is simply a group of people that know each other ahead of time, and can contact each other through email, twitter, facebook, or some other means common to the group.
For example, I could post here on F.R. that all FReepers in the D.C. area should meet under the Washington Monument at noon tomorrow, and unless your 'service' actively monitored Free Republic, they'd never catch it.
Some of the hoodlum flash mobs are probably arranged through cell phone text messages, so no private company could possibly monitor that activity.
I read about a program called TXTMob. Wonder if thats still being used.