It’s been a big few days for location-based service Foursquare, which just hit 3 million users and also became the centerpiece of this gigantic advertising ziggurat in Times Square. It’s unclear what kind of deal made this ad happen, since it seems unlikely Foursquare could afford such a high-profile placement. On his blog, Foursquare’s Tristan Walker thanks the sign’s owner, apparel company American Eagle, but also says the ad was designed by Foursquare’s own designer, Mari Sheibley (aka, the woman who draws the badges). If it’s true, as reported by Mashable, that no money changed hands, this could be a sign of a growing partnership between American Eagle and Foursquare, or that American Eagle sees a lot of value in getting more people checking in (the retailer offers a 15 percent discount for Foursquare users). Either way, it’s a huge publicity boost for a service with competitors ranging from the scrappy Gowalla to Facebook, a site whose users outnumber 167 to 1.