Zuckerberg said on Wednesday under questioning by U.S. Representative Ben Luján that, for security reasons, Facebook also collects “data of people who have not signed up for Facebook.” Facebook gets some data on non-users from people on its network, such as when a user uploads email addresses of friends. Other information comes from “cookies,” small files stored via a browser and used by Facebook and others to track people on the internet, sometimes to target them with ads.

Facebook's Tracking Of Non-Users Sparks Broader Privacy Concerns