title: passport-facebook, serializeUser flow
body: <p>Ive been studying express, passportjs and passport-facebook. The thing is, I dont know the flow of it and in the documentation I found nothing (IMO the documentation is poor). For example:</p>
  
  <ol>
  <li>When is serialize and deserialize called? Every request? Or serialize is just called the first time when you log in? </li>
  <li><p>In the serializeUser function, where does the user parameter come from?</p>
  
  <p>passport.serializeUser(function(user, done) {
      done(null, user._id);
    });</p></li>
  </ol>