title: Facebook tracking who Invited whom
body: <p>I have a Facebook app.  Lets say user A invites user B, and user C also invites user B to use a Facebook app.   </p>
  
  <p>User B clicks on user C's invite and accepts it. </p>
  
  <p>How do I check this? I'd like to assign the referral to user C.</p>
  
  <p>I can't find anything on Facebook forums.</p>
  
  <p>PHP SDK</p>