title: How to check if a user is logged into facebook using his access_token and graph api
body: <p>I have a facebook iframe application.
  I am doing my requests to facebook on server side using the graph api.
  I have offline access so the access_token never expires.</p>
  
  <p>I would like to know if the current user is logged into facebook so when he logs out of facebook i can log him out of my application.
  I do not want to use the javascript sdk.</p>
  
  <p>I have tried using the online_presence field of the user table but that always returns null.
  Is there any way i can do this?</p>
  
  <p>Is there a better way about it?</p>