Most people know that Facebook has information about them. We submit things like our names, our hometowns, our ages, our birthdays and our interests, and we assume that Facebook is collecting that data. But Facebook has much more data on most people than they realize. Facebook can take all the data that you submit and combine it with data from other users and outside information to construct a profile of you. Facebook uses nearly 100 different data points to classify your interests and activities. This would include basic stuff like your age and gender, but also more complicated information like whether you own a motorcycle or you recently went on vacation or whether you’re a gadget geek. Researchers have found that by using signals such as your likes and interactions, Facebook could tell if you were in a relationship or going through a breakup. Facebook doesn’t just know who you are. It also knows where you are. If you have location tracking turned on, Facebook collects an enormous amount of location data about where you’re going, where you came from, where you live, where you work, what restaurants and businesses you tend to go to. And they use this information to target ads at you. And location data could reveal other people who live in your house, even if you’re not connected to them on Facebook. Now obviously, Facebook knows what its users buy when they click on ads from Facebook. But what most people don’t realize is that they have ways of tracking your offline purchases as well. For many years, Facebook has had partnerships with data brokers that collected information about people’s purchases. So for example, if you buy a burrito with your credit card, Facebook could know about that transaction, match it with a credit card that you’ve added to Facebook or Facebook Messenger, and start showing you ads for indigestion medicine. One of the most controversial parts of Facebook data collection is a feature called “People You May Know.” And this is where Facebook uses many different signals of what it knows about you to determine who else you might be connected to. And this is not always things that you share with Facebook. It might be contacts in your phone. It might be people who have been in the same room as you. Facebook was using location data to recommend friends. So it might have been recommending people who share a doctor with you or work in the same building. Facebook can also be used to compile data about your political activity like protests or marches you go to. In one case in 2016, the A.C.L.U. found that 500 police organizations had signed up for a service called Geofeedia, which scraped data from social networks like Facebook, Twitter and Instagram to help officers look for users who might be in a specific location or attending a specific protest. For example, Geofeedia claimed it helped the Baltimore Police Department monitor and respond to the protests after the death of Freddie Gray. Facebook doesn’t just know who you are, where you are and what you buy. It also can be used to figure out what kinds of things you might do in the future. To predict life outcomes, like whether you will be addicted to substances, whether you will switch political parties, whether you’re physically healthy or physically unhealthy. These are all part of the information that advertisers love to know because it helps them better target users.