Illustration: Rebecca Zisser/Axios

On Facebook's map of humanity, the node for "you" often includes vast awareness of your movements online and a surprising amount of info about what you do offline, too.

The big picture: Even when you're cautious about sharing, Facebook's dossier on you will be hefty. Facebook tackles its mission of "bringing the world closer together" by creating a map of humanity, and each of us represents a tiny node on this "social graph."

Assembling your profile: This is where your Facebook presence begins.

  • When you create an account, Facebook asks for your name and birthdate, along with either a phone number or e-mail.
  • Then there's all the information you give Facebook as you fill out your profile, potentially including schools, current and past occupations, relationship status, hometown and current city, as well as your physical address, birth name, website, and other social links.
  • All of this forms the core of the profile Facebook uses to serve you ads. It's why you see offers for clever T-shirts based on your college or job.

Following what you do on Facebook: The company has near-total awareness of every move you make on its website or in its apps, including:

  • When you log in, how long you spend online and where you are logging in from — hence it can welcome you to new cities and suggest places to visit and eat (and also serve up local ads).
  • Places you check in.
  • The pages, accounts and hashtags you connect with on Facebook — and not just who you are connected with, but how often you interact and for how long.
  • Your contacts, if you choose to upload your phone book or call history.
  • Things you buy directly from or through Facebook, but also things you may not think about, like the metadata from photos you upload.
  • Your friends can tag you in posts and photos, which gives Facebook additional information. (You can choose to have this displayed publicly or not via privacy settings.)

Following what you say on Facebook Messenger: Facebook does scan your chat messages, but it isn't exactly reading them — it runs an automated scan for child pornography and other banned content.

  • Messenger can collect information on who you talk to, how often and for how long, as well as phone history if users opt in. But the company says it isn't serving ads based on the content of users' messages.
  • It also has an option for users to encrypt their messages, but this is turned off by default.

Following you outside Facebook: Facebook sees you less thoroughly outside its own digital turf, but it still sees a lot. This data comes from two places: partner services and third-party information brokers.

  • Facebook has tools that partner websites use to integrate with Facebook, including the inclusion of "Like" and "Share" buttons, as well as a tracking cookie known as Facebook Pixel.
  • Thanks to an inquiry from Britain's Parliament, we have a sense of how prevalent these methods are. According to Facebook, between April 9 and April 16 of 2018 there were 2.2 million Facebook Pixels, 8.4 million pages with a Like button and 931,000 pages with a button to Share on Facebook.
  • Facebook knows your location, even if you haven't directly given it permission to access your phone's GPS, by tracking the IP address of the phones, computers and other devices you use to access its servers.
  • Facebook also reserves the right to enhance its data trove by adding information from outside providers, though it has ended one program that mixed Facebook and third-party data for advertisers. From its policy page: “We also receive information about your online and offline actions and purchases from third-party data providers who have the rights to provide us with your information."

Following you across your apps: Many apps are connected to Facebook, including through its popular Facebook Login feature, which uses your Facebook account as a shortcut for you to sign in.

  • Developers can also use this system to get your permission to access Facebook data. In addition to iOS and Android, it also works across the web and on some smart TVs.
  • Integrating Facebook was once a way for outside apps to get a lot of info about you, but Facebook has tightened that up considerably, setting rules and instituting a review process for apps that want anything beyond basic identity information.

Following you at home and around town: Facebook's new Portal video chat system is basically a camera that lives in your home.

What Facebook does with all this data: Facebook says, emphatically, that it doesn't sell your information.

  • It does use the data to sell you to advertisers who set criteria for people they want to target. The more the company knows about you, the more valuable those advertisements can be.
  • It also uses the information to enhance its social graph, which it uses to build new features and products, and to power its suggestions of "People you may know."

What Facebook doesn't know about you: Facebook insists it doesn't monitor your phone calls or secretly record you via microphone, despite long-running suspicions to the contrary.

The bottom line: Facebook's privacy policies reinforce the message that "you have control over who sees what you share on Facebook." But if you use Facebook at all, you don't have much control over what Facebook itself sees about you.

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