Facebook’s ad platform translates your clicks and posts into “user attributes,” and places you in certain categories for advertisers. For example, your activity could suggest an that you recently had a child, that you’re a Bernie Sanders fan, or that you have an “affinity” for certain racial groups. One of the most powerful features is called “Custom Audiences,” which allows an advertiser to target individuals based on offline information such as email address or phone numbers. An advertiser can upload that data to Facebook, which will match them with its user base to find who they belong to. Then there are so-called “dark posts,” a feature Facebook is in the process of changing, which only show up in the news feeds of specific audiences and are completely hidden to everyone else.