Flash mobs are not a recent phenomena. The Wobblies were there first and arrived without benefit of text messages. When one of their members (like Orrin Pappas here) was arrested for exercising his right to free speech, word spread like wildfire, and hundreds of Wobblies would show up at once. Since free assembly was also a crime, they'd all be rounded up and jailed as well. The costs to cities and towns was so great, the police would have to release them, but they would refuse to go until all were freed. A highly effective strategy, if not exactly as much fun as a zombie crawl or a pillow fight.
Happy Labor Day.