If you have a Facebook account, then you know the deal: you get to connect with your friends, family, loved ones, and those people from high school you never talk to, all for free. In return, Facebook collects information about you—your profile information, articles or pages that you “like,” videos you watch, and so on—and uses that to sell ads.

But it’s not that simple. As an ongoing investigation by ProPublica has shown, Facebook is going beyond the tacit agreement that it provides a free service in exchange for online personal information. It has contracts with several data brokers that provide Facebook with information about your offline life—things like how much money you make, where you like to eat out, and how many credit cards you keep.

It is using that data to flesh out its advertising profile of you, and it isn’t telling you about it.

Facebook’s advertising operation is a remarkable machine. Sure, the social network has an extraordinarily large user base, but what really turns advertisers on is that it lets marketers narrowly define the subset of users who will see an ad based on all sorts of parameters, including users’ shared interests, political leanings, age, and mobile devices.

That kind of microtargeting is incredibly valuable—and what better way to augment it than to buy up offline data sets that can then be matched to Facebook’s users? Far better than simply knowing that someone clicked “like” on the Food Network’s page, for example, is knowing how much money they make each year, or whether they shop at low- or high-end retail stores.