Facebook扩展访问令牌不起作用

I'm using the read_stream permission to get comments,likes and shares info by a user on a particular page. For that I need a long lived access token as I need to poll the servers and update them periodically. But, when I do this:

  if($user){
   $user_profile = $facebook->api('/me','GET');
    $facebook->setExtendedAccessToken(); 
    $access_token=$facebook->getAccessToken();
 function xy(){
xx...
}

I always get only a token that expires in one hour.

Is there something I'm missing? I'm using the PHP SDK