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In response to the boom in usage, the developers added a new badge called Super Swarm. There was already a badge called Swarm; it was awarded when 50 people checked in at the same location at the same time. Super Swarm is achieved when 250 people check in.
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Foursquare guru Harry H. said: "Know what you call a 50 person swarm at SXSW? The Hilton Lobby. So we upped the ante to 250 and you still nailed it. Well played!" So yes, the badge has already been won, but it's not a zero-sum game; head to a huge event like SXSW and you might get it, too.