The senseable way would be to add a field that has userposts. When a new user gets in there mark it as 0. Then when they make a post have it add 1, Update the table, Add 1 Update table, ECT..ECT.. Then you can search for it by saying , Assuming that your field is name userposts.
"Select * from tablename order by 'userposts' DESC";
Check the syntax on it... it might not have a ' ' in it. I can't remember. That is simply what I would do. There are probally better ways to do it with a script. I don't know.
Ziggs