Pure flash mobs, like the Grand Central caper, are organized via social media, on short notice, for no apparent reason. Think dozens of people in a crowded subway car, talking into bananas. The newer stunts are often polished productions billed as "random acts of culture." The concept already has been co-opted for commercial purposes (most famously by T-Mobile), to hype upcoming events or to raise money. There have been mobs to protest the BP oil spill and to demand that Chicago's mayoral candidates articulate their arts platforms. ... It's going to take some imagination to keep this trend from going south fast.