title: Facebook graph API, private messages: Difference between threadIds starting with "t_mid." and "t_id."?
body: <p>I have a crawler for Facebook private messages on behalf of our customers, which works fine most of the time (99.99%). To answer on these crawled messages I create an url, which corresponds to the "location" of the message. Sometimes the creation of a proper url fails because of a bug in my system. The bug was related to the assumption that a threadId always starts with "t_id." which isn't true. Some threadIds are starting with "t_mid." and I have not the faintest idea why.
  My bug is easy to resolve, but my curiosity isn't:</p>
  
  <p>What is the difference between Facebook Ids with these format?</p>
  
  <ul>
  <li>t_id.############ - ThreadId</li>
  <li>t_mid.############:SomeHexValues - Also ThreadId? </li>
  <li>m_mid.############:SomeHexValues - Unique message id</li>
  </ul>
  
  <p>It seems to be customer related; Most of our customers do have the "t_id" format, only some of them have the "t_mid" format.</p>
  
  <p>I did not find anything related on the documentation at developers.facebook.com, but maybe someone has an idea here.</p>