Retrieving posts and comments from a Facebook page

Question

I want to retrieve all of the posts from a given facebook page along with the associated comments.

I wrote this code (app details obfuscated, replace with your own in order to run it).

<?php
require_once('facebook.php');
$facebook = new Facebook(array(
    'appId'  => 'MY_APP_ID',
    'secret' => 'MY_APP_SECRET',
    'cookie' => true,
));

$pages = array(
    "stackoverflow" => 11239244970
);

$result = $facebook->api(array(
    'method' => 'fql.multiquery',
    'queries' => '{
        "posts": "select post_id, source_id, actor_id, target_id, likes, message from stream where source_id = '.$pages["stackoverflow"].'",
        "comments": "select post_id, text, username, fromid from comment where post_id in (select post_id from #posts)"
    }'
));

echo json_encode($result);
?>

posts returns the expected results, but comments returns just one comment.

This example queries the stackoverflow facebook page

The comment returned from the comments query is "Joined!" (from this post). I can't figure out what's special about this comment.

Any thoughs?