Most of these mobs are not from social networks, but from text msgs, so I think only the NSA has the technology needed to do what you are asking.I doubt they’d use that tech for this problem, and even if they did a real 4th amendment question would arise..I think the best course is back to basic beat cop techniques.Observation, CIs,citizen tips.Your suggestion about an alert system, akin to Amber Alerts is one that could be done once the info is verified.
then again, why should law abiding citizens have to be “warned” about flash mobs....we have every right to live peacefully .....flash mobs better start getting the “warning”, not us...