I am using the FB graph api to get a list of my friends\' friends using:
https://graph.facebook.com//friends?access_token=
I got friends of friends(limited). I had same problem. Though it is very late for answering question, it will help somebody. That's why answering this question.
We can get friends of friends those are app users. Try by this fql query.
$fb_id= user id whose friends of friends required.
$query="SELECT uid, name, work_history FROM user WHERE uid IN (SELECT uid2 FROM friend WHERE uid1 IN (SELECT uid FROM user WHERE uid IN (SELECT uid2 FROM friend WHERE uid1 = $fb_id ) and is_app_user=1) )";
$user_info=$facebook->api(array('method'=>'fql.query',
'query'=>$query));