Businesses can now tell if a Facebook ad led you to shop at their stores

Facebook's advertisers now have a better understanding of how much of your mall shopping might have been inspired by their ads.

The social network just rolled out a new feature that lets advertisers include an interactive map of nearby brick-and-mortar stores within their carousel ads — those slideshow-like promoted posts that must be swiped from side to side to view.

Facebook can then track whether people who look at these ads on mobile actually visit the stores featured and hand that information over to advertisers. That is, as long as it has a user's permission to collect data on his or her location, which can be enabled in a smartphone's settings.

In some cases, it will even follow your shopping all the way to the check-out counter. The final piece of the update is a new API that lets businesses match their in-store purchase data with Facebook's advertising measurement tools. 

That will not only tell companies how many of the people who saw their Facebook ads bought something in their stores but also give them a better sense of the demographic breakdowns of customers.

Physical shopping has long been a blindspot in determining the effectiveness of online ads.

Even as e-commerce grows more popular, online sales still accounted for less than a tenth of all purchases in the United States in the first quarter of 2016, according to a May report from the Census Bureau.

The vast majority of people still do most of their shopping at brick-and-mortar shops, and of those who do shop online, most prefer to do it on desktop rather than mobile.

Online advertising giants like Facebook and Google know this. That's why they've been looking for new ways to take advantage of the ability to track your location on mobile to better understand how people spend their money off the web and to drive people into physical stores.

Google rolled out a similar ad product last month that lets users search store inventories and companies promote their store locations on its Maps feature. Like Facebook, The company also tracks visits to stores after ad viewing.