The new leaderboard ranks brands by number of followers. It also indicates how many, if any, custom badges a brand's Foursquare account offers to users. The leaderboard is updated every few minutes. It's an interesting way to gauge what types of brands are gaining the most followers.
Right now, in addition to Zagat, Bravo, The History Channel, MTV and Bon Appetit Magazine round out the current top five Foursquare brands.
In fact, just looking over the top 25 brands, most can be segmented into one of three categories:
TV Network or TV Show
Newspaper, Magazine or News Website
City Guides
Brands that highlight local finds, food or general culture tend to have a lot of followers, but the vast majority of the "biggest" Foursquare brands appear to be bigger media companies or brands (including Mashable, which ranks at #15) with audiences that tend to be early adopters and tech savvy.