The original Swarm badge was created to reward users who checked into a venue with 50 different users. Super Swarm — for 250 users — was created after the Swarm badge was deemed simple to obtain. And now, thanks to the increasing popularity of the service, Foursquare had to introduce two more crowd-based badges.
Generally, it's fairly easy to unlock even the Super Swarm badge at a concert, sports game, or other major, crowded event where there's likely to be many Foursquare users. I'm curious where the first Epic Swarm will be!