SELECT u.username,
COUNT(*) AS monthlypostsers
FROM xf_post p
LEFT JOIN xf_user u ON (p.user_id=u.user_id)
WHERE p.post_date BETWEEN UNIX_TIMESTAMP('2012-08-14 00:00:00') AND UNIX_TIMESTAMP('2012-09-14 00:00:00')
GROUP BY p.user_id
ORDER BY monthlypostsers DESC;