Flashmobs are a new social phenomenon that is hot and growing. They are gatherings of dancer participants at large public places. The one shown below looks like it's on the verge of utter chaos. Participants bring their MP3 players, dance and video/photograph the event and push it back to their friends on the SM (social media) channels. No doubt other advertisers like T-Mobile trying to figure out how to coopt the events. T-Mobile got it all started. See this link - T-Mobile Station Dance.