Using Facebook\'s Graph API, given a username xyz (assuming they\'ve authenticated my site), how do I get a list of all of the facebook pages that the user administers?
After getting access token you can get all details of list of all of the facebook pages that the user administers.
https://graph.facebook.com/[FACEBOOKUSERID]?metadata=1&access_token=
The output will look like
{
   "name": "Facebook Developer Garage Austin - SXSW Edition",
   "metadata": {
      "connections": {
         "feed": "http://graph.facebook.com/331218348435/feed",
         "picture": "https://graph.facebook.com/331218348435/picture",
         "invited": "https://graph.facebook.com/331218348435/invited",
         "attending": "https://graph.facebook.com/331218348435/attending",
         "maybe": "https://graph.facebook.com/331218348435/maybe",
         "noreply": "https://graph.facebook.com/331218348435/noreply",
         "declined": "https://graph.facebook.com/331218348435/declined"
      }
   }
}