After being restricted to just a handful of cities for the first several months of its existence, Foursquare is now in a period of rapid expansion around the globe. Fresh of its launch of 15 European cities a couple week ago, today the site has rolled out support for 50, yes 50, new cities. Though they haven't formally announced the massive expansion yet, which pretty much doubles their total (they had 53 cities previously), Foursquare's Harry Heymann tweeted about the addition today. And if you look in the site's city drop down list, you'll see a huge list of new cities.