With an audience of around 1.09 billion daily users, Facebook has become one of the most wide-reaching advertising platforms in the world, and the wealth of social data that comes with it is something that marketers from a couple of decades ago would probably have killed for. Now, Facebook is about to add one more valuable metric that could completely transform the advertising industry: whether or not an online ad actually gets a user to go out and visit a store.

Facebook has said that it will now monitor what stores you visit by looking at the location data from the Facebook app, and it will be able to see if you recently viewed an ad for that store on Facebook. Obviously, this could have a huge impact on the way businesses use Facebook to advertise.

“This is a window into a black box that they’ve never had before,” Sam England, Facebook’s product manager, said in an interview with AdWeek. “It’s not just something new for Facebook but something across the board.”

“This is one of the biggest partner challenges that exist in a digital and mobile world,” added Maz Sharafi, Facebook’s director of monetization product marketing. “Consumers are increasingly spending their time in mobile and online, but transactions are happening everywhere.”

Of course, while this data would clearly be useful to advertisers, it also adds yet another line to the long list of privacy concerns surrounding Facebook, especially since this information would be shared with third-party marketing companies.

According to Facebook, however, all of the data will be anonymized, so while ad sellers will be able to see that a user viewed an ad and then visited a store shortly afterward, they will not be able to see any personally identifiable information about that user. Users can also opt out of being tracked by disabling location services on the Facebook app.

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