You must have used Facebook login across different websites and apps many times. But this convenience of having one common login for all your favourite apps and websites comes with a massive security risk. There are some third party scripts on some of those websites/apps that you login via facebook , that can potentially scoop up your personal profile data like name, profile picture, emails, location, profession etc when you try to login through Facebook’s Login button without your permission.
Technically when you click on Facebook Login, then website is supposed to get your personal info from Facebook, but sometimes there are third party scripts present on those websites who scoop up your data as well.
Once the third party script access’s your facebook profile data, it can then use to track the user across the websites, devices etc.
If you want to protect your data, then be extremely cautious of using Facebook Login on websites and at the same time install trusted ad blockers to prevent any unsolicited data breach.