title: Facebook Graph API private_replies with page access token throws an error
body: <p>I'm trying to send private replies as a page with the Graph API:</p>
  
  <pre><code>FB.api( '/' + id + '/private_replies', 'post', 
  
      { message: message, access_token: token }, function(res) {}
  );
  </code></pre>
  
  <p>The comment id is fine. The access token has been generated correctly. I have read_page_mailboxes permission. Despite all that, the API throws back this:</p>
  
  <pre><code>{
    "error": {
      "message": "(#10903) This user cant reply to this activity",
      "type": "OAuthException",
      "code": 10903,
      "fbtrace_id": "BW3yOdmwnhi"
    }
  }
  </code></pre>
  
  <p>Am I missing something?</p>