Of late, I have been visiting some sites like Yelp. I noticed that even though I did not explicitly log into the site using "Facebook Login" button, it automatically logs in using that profile. (I have may sometime earlier given it permission to do so.) Even though it is a different site, how is it able to pick up the Facebook login data from a different cookie?
Facebook – How to use Facebook cookie on other sites
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Best Answer
They use Facebook's JS library to authenticate and use the available data to sign in to their website. The cookie is not shared. Sites like Yelp may use the data provided by the JS library to create new session/cookies and use that to detect if you've accessed that site later on. More details can be found in Facebook's documentation.