Чтобы получить последние ответы, попробуйте это
SELECT text, time, id, likes, fromid, comments FROM comment WHERE is_private = 0 AND
object_id in (select post_fbid from comment where object_id in (select comments_fbid from link_stat where url ='YOUR_URL'))
ORDER BY time DESC limit 0,10
Чтобы получить оба комментария и ответы, попробуйте следующее:
SELECT text, time, id, likes, fromid, comments FROM comment WHERE is_private = 0 AND
object_id IN (SELECT comments_fbid FROM link_stat WHERE url ='YOUR_URL')
or
object_id in (select post_fbid from comment where object_id in (select comments_fbid from link_stat where url ='YOUR_URL'))
ORDER BY time DESC limit 0,10
ОБНОВЛЕНИЕ : поле комментариев удаленоиз таблицы комментариев.