How can one securely log users in automatically after previous Facebook authentication?

Question

Users are complaining they have to login too frequently. If authentication is entirely built on Facebook OAuth, how can a user be logged in automatically the next time they

Answer 1

Make use of the JS-SDK FB.getLoginStatus() method:

FB.getLoginStatus(function(response) {
  if (response.authResponse) {
    // logged in and connected user, someone you know
    // reload page or use ajax to update content of page
  } else {
    // no user session available, someone you dont know
  }
});