Is there any way to know which member made the xth post, say for example the 1000th post or thread? Does it have to be run as a query from the phpyadmin or is there any other way from the ACP?
you can check it from the forum like this :
http://community.mybb.com/showthread.php?tid=1000
http://community.mybb.com/showthread.php?pid=1000
That won't necessarily be the 1000th, as if posts or threads get deleted, the numbers won't get reused.
The best way is to run a query:
SELECT `pid` from `mybb_posts` ORDER BY `dateline` ASC LIMIT 999, 1;
SELECT `tid` from `mybb_threads` ORDER BY `dateline` ASC LIMIT 999, 1;
Change the 999 to the number that is one
less than the number you're trying to find, i.e. 999 will find the 1000th post/thread. Then go to yoursite.com/showthread.php?pid=X (post) or yoursite.com/showthread.php?tid=X (thread) where X is the number returned by the query, and that will be the actual 1000th post/thread.