I have never participated in or observed a flash mob, but I have seen several from videos or movies. The one flash mob that impresses me the most is from the American show <<Shameless>>.
The college that our teen protagonist attends has a tradition where every student stops their motion for 10 seconds at a pre-specified time every day. This is unusual since every student has to stop doing whatever they are doing and just freeze as if the time has just stopped. I totally do not get the meaning behind such flash mobs (or there is simply none), but It seems that the very action of gathering groups of people and commanding them (whether or not there is an actual organizer) to perform certain actions together, is itself an action of art. Just like the guy who attempted to have Columbians line up their boats across the ocean to reach United States, the power of cooperation can be considered an artistic achievement to me.