"If you want to log into a website using Facebook login, Facebook needs to know what website you are logging into."
Agreed. But what if you don't want to log into that website, or maybe that site has no login at all. If it has a Facebook "Like" button, that site still sends a ping back to Facebook, letting them know you were there, and feeding Facebook's algorithm about your interests. Same goes for Google Analytics, a Twitter share button, or the growing list of tracking scripts which get executed upon page load without even showing a visual indicator that they serve some purpose.
I'd guess that the vast majority of visitors have no idea that by visiting a 3rd party site, they're feeding that visit to the list of 3rd party trackers you find on many sites today.
Agreed. But what if you don't want to log into that website, or maybe that site has no login at all. If it has a Facebook "Like" button, that site still sends a ping back to Facebook, letting them know you were there, and feeding Facebook's algorithm about your interests. Same goes for Google Analytics, a Twitter share button, or the growing list of tracking scripts which get executed upon page load without even showing a visual indicator that they serve some purpose.
I'd guess that the vast majority of visitors have no idea that by visiting a 3rd party site, they're feeding that visit to the list of 3rd party trackers you find on many sites today.