Flash Mob
Flash Mob: a large group of people who assemble suddenly in a public place, perform an unusual action for a brief time, then quickly disperse. The organization of flash mobs is generally accepted as being limited to the social media or viral emails, rather than organized by public relations firms or for a stunt. Also flash mobs are not politically or commercially motivated as described by Bill Wasik’s in where he said, they serve no purpose (wikipedia)
Recently I’ve been very much entertained watching flash mob videos in YouTube after watching a T-mobile flash mob gig. Though technically, based on the flash mob definition, this is not a flash mob because it was a stunt for a T-mobile ad, but nonetheless it was very much entertaining. As the music progress you can see more and more people “joining” the dance 
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I agree with you.