вам нужно сделать дополнительный запрос, примерно так:
$multiQuery_postInfo = array(
"posts" => "SELECT post_id, actor_id, message, message_tags, likes, comments, share_count, attachment, action_links, type FROM stream WHERE source_id=$pageID",
"users" => "SELECT uid, username, first_name, last_name, profile_url, pic FROM user WHERE uid IN (SELECT actor_id FROM %23posts)"
);