title: Why should we trust someone authenticated via e.g. facebook?
body: <p>When using OAuth we are using a third party to take care of the authentication of a user.<br>
  E.g. when using Facebook we “register” our app with Facebook and when a user of our app tries to login instead of providing to the app his credentials he logs in using his Facebook account.<br>
  What I don’t understand is why does the app trust the user? All the app knows is that the user is trusted from Facebook as he has an account there. How can we trust that the user should really access the app?</p>