Facebook will now let advertisers put digital ads in front of people who previously visited their real-life stores or those of their competitors.

Although companies like Google offer offline measurement that helps marketers figure out whether their online ads led to physical store purchases, Facebook is now serving ads to people based on their physical travels. Its advertisers can also buy against larger groups of so-called lookalike audiences that resemble physical-store visitors in certain ways , or exclude people who have already visitied certain retailers -- for promotions aimed at new customers, for example.

Large Facebook advertisers can also serve ads to people who actually made offline purchases, as well as to their lookalikes.

Initial buyers include KFC, Dick's Sporting Goods and Macy's, according to Facebook.

"This allows businesses to close the loop on their advertisements," says Gabriel Francis, product manager of offline sales at Facebook. "Twenty years ago, during the advent of online advertising, we saw that people didn't just want to understand the impact of their digital ads on individual purchases. They also wanted to use that information so they could optimize their ads in order to find their best customers and find the optimal conversion rate and then use that information to improve their advertisements so they could be really efficient with their ad spend."

While companies like NinthDecimal, PlaceIQ as well as Snapchat's recent acquisition -- Placed -- offer similar services, Francis argues none of them have the reach of Facebook's 2 billion users.

Facebook will still have to convince marketers that its targeted ads were responsible for any purchases that follow, a familiar challenge in advertising that isn't made any easier by going after people already visiting stores. Someone who visits Best Buy to check out a top-tier TV set, for example, may have been planning to buy it in the near future regardless, targeted ads or not.

Francis says Facebook's Lift platform should be able to let marketers determine the impact.

It's unclear how accurate those sorts of analytics really are, given the many other factors in play. But KFC doesn't appear to lack confidence in the results. The fried chicken-chain said it was able to increase consumer spending per store visit anywhere from $.50 to $2 per store visit.

Tom Hassett, VP of media at Dick's, says the sporting goods retailer tested the product for weeks. It used it both to target people in winter who visited its stores in summer and to build lookalike audiences to drive new people into stores.

Facebook finds out where its users are going by accessing data from people who have the Facebook app with location services enabled on their phone.

"We use that information to understand the baseline and then extrapolate up in order to bring it in line with Facebook's user population," Francis said. "So, it's an estimate of people who have location services enabled and it is primarily first-party data that we use."