I don't really think that it is so much MySQL being unable to scale. I think the problem is in the dbforum.com configuration of MySQL.
While MySQL isn't likely to ever compete with DB2, MS-SQL, or Oracle, it can handle significant jobs and do them well. Like any open source tool, the cost in MySQL is in the ongoing maintenance and the planning/management as it scales up. I don't feel that anyone has invested much (certainly not enough) time and energy in making MySQL work for dbforums.
Truth is, I do not think we will ever really know what is going on with the site. We could volunteer to go over their systems, but I don't know any admin that would admit to not being able to troubleshoot their own systems. It is part of what makes a good admin. The need to find and solve a problem. Besides. I am a SQL Server guy. I know almost nothing about MySLQ, except how to spell it ;-).