As explained on that shiny new portal, Facebook keeps ads “useful and relevant” in four distinct ways. It tracks your on-site activity, such as the pages you like and the ads you click, and your device and location settings, such as the brand of phone you use and your type of Internet connection. Most users recognize these things impact ad targeting: Facebook has repeatedly said as much. But slightly more surprising is the extent of Facebook’s web-tracking efforts and its collaborations with major data brokers.
While you’re logged onto Facebook, for instance, the network can see virtually every other website you visit. Even when you’re logged off, Facebook knows much of your browsing: It’s alerted every time you load a page with a “Like” or “share” button, or an advertisement sourced from its Atlas network. Facebook also provides publishers with a piece of code, called Facebook Pixel, that they (and by extension, Facebook) can use to log their Facebook-using visitors.