I have visited a site . I have observed that When I am not logged in Facebook it ask for connect to facebook to download file. After that I have authenticated their Application. Now Whenever I logging in to Facebook and open this site it automatically detect that I am login in Facebook without any authentication.
I want to know that how it can identify that I am logged in Facebook? Can anybody suggest any good artical which shows the workflow of FaceBook conntect authetication behind the scene?