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After checking in via Facebook Places (which can be done through Facebook's official iPhone app [iTunes link] or by visiting touch.facebook.com if you're on another web-enabled device), participants will need to show their checkin to a Gap employee for a chance at a free pair.
This is just one of a number of mobile and social media-based campaigns Gap has launched in recent months to get shoppers into its stores. In August, Gap partnered with daily deals site Groupon to offer $50 worth of Gap merchandise for $25, selling 441,000 Groupons in total and smashing previous sales records in the process. Gap also offered 25% discounts to all shoppers who checked in to a retail location via location-based social network Foursquare that same month.