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!
Simply checkin to a location that 250+ other people have already checked into in the last three hours.
Info: This badge was originally only attainable during SXSW 2010. Visit the SXSW foursquare page for more information.