FTP users are just in the database, there are no commands executed when you create a FTP user.
Also do not mix up shall and FTP users. You canm not login with the shell users with FTP. You have to use a ssh client like putty to login with shell users or use sftp with a client like winscp.
if you want to debug your FTP logins, enable debugging in pure-ftpd:
and you can enable it also in ispconfig: