[Note - it seems that they boosted the number of checkins by 33%, rather than the total footfall]
Econsultanty reports that McDonald’s, with Wion driving campaign direction and strategy, opted to try and take advantage of Foursquare Day (4/16) to bring in more business. The company used 100 randomly awarded $5 and $10 giftcards as checkin bait to lure in potential diners. The bait also worked to attract the media’s attention and resulted in more than 50 articles covering McDonald’s Foursquare special.
The campaign worked in both digital and real world capacities. Patrons flocked to McDonald’s restaurants for the chance to win giftcards in exchange for checkins, and 600,000 online denizens opted to follow and fan the brand on social media sites.
“I was able to go to some of our marketing people — some of whom had never heard of Foursquare — and say, ‘Guess what. With this one little effort, we were able to get a 33% increase in foot traffic to the stores’,” Wion explained to conference attendees."
Source: Rick Wion of McDonalds, speaking at the Mobile Social Communications conference, reported by Mashable, 17th September 2010
Note - Always treat company's own pronouncement of effectiveness (especially at conferences) with a degree of caution!