If you've ever opened an app and selected the option to "log in with Facebook" instead of creating a new account, chances are that the people or company behind that app know a lot about you.
According to Facebook, each app that you log into will get your gender, networks you belong to, username, your user ID, your full name and your profile picture. They also get access to your full friends list and any other public information on your profile.
And you may have used this method to log into an app dozens or — as I discovered on my account — even hundreds of times.
This issue is at the center of the Cambridge Analytica data scandal that the social media company is facing right now. The political research firm was able to access data on more than 50 million Facebook users through a third-party personality quiz application, without the knowledge or consent of users. The quiz maker wasn't supposed to pass the information along, but Facebook at the time had no technical measures in place to prevent it. Cambridge Analytica later used this and other information to create detailed psychological profiles of Facebook users and to micro-target political ads at some of them, according to recent reports in the Observer and The New York Times.