According to a report released by LocalResponse, a New York company that helps companies target customers based on social media sentiment and check-ins, Walmart captures nearly 40% of mobile check-ins via Foursquare, Twitter, Instagram, Pinterest and Yelp among the top ten U.S. retailers.

The reason: sheer volume. In terms of sales and retail footprint, reliable proxies for foot-traffic, Wal-Mart is in a category of its own. The company’s 4500 U.S. stores generated $67.4 billion in sales last quarter. That’s over $30 million an hour and half a million every minute. By comparison Kroger, the second largest U.S. retailer by sales, rakes in about one-third of Wal-Mart’s quarterly revenues.

LocalResponse CEO Nihal Mehta reflected that Walmart’s dominance of mobile check-ins shows that smartphone penetration is no loner a blue-state phenomenon. “It’s now a mass market,” he says. “You can buy a smartphone for $50 these days.”

Over the course of the summer, LocalResponse tracked the public mobile check-ins of hundreds of thousands of users across five social networks. The company found that people check in the most over the course of the weekend, with 25.9% of location data registered on Saturday followed by 19.7% on Friday and 14.3% on Sunday. According to the company’s report, attached below, Saturday at 2pm ET is check-in “primetime” in the U.S. More than three-quarters of the location data captured by the study are from Foursquare, while Instagram placed last with just a 1.4% share.

Despite its second place position among top retailers by sales, Kroger ties for last place with Safeway in terms of share of mobile check-ins.  The likely culprits are demographics and social media perceptions. It’s not exactly cool, even ironically, to post a picture of raw chicken on Instagram.

Men, it seems, are more inclined to share their location over the social web, accounting for 54% of the data collected over the measurement period. Not surprisingly, males accounted for outsized portions of check-ins at electronics retailer Best Buy and the home-improvement chain Lowes (67% and 65% respectively).

The data shows that Target punches above its weight, capturing 15.4% of social media check-ins with roughly one-fourth of Walmart’s quarterly sales. The company attracts a younger, higher-income demographic than Walmart which likely increases its social media appeal. Fifty-three percent of check-ins at Target came from women.

LocalResponse helps advertisers like Coca-Cola, GM and Walgreens quickly respond to consumers who have mentioned their products or industries, or checked in nearby, over social media. If I tweet about how hungry I am for pizza, for example, LocalResponse can help Pizza Hut track that message and respond directly with a coupon.

Moving into the holiday season, the company is helping retailers analyze historical social media data to retarget customers who checked into their stores, or even those of their competitors, during Black Friday in 2011.