This is a common pitfall when people start using Memcache. Memcache puts less load on the DB, which means you can keep the same DB hardware and scale up your pageviews...until Memcache craps out and all that load is now back on the DB again :(
Since most of their (original) codebase is/was PHP, I would say there is a high likelihood that their db is MySQL (though I guess you could refer to it as Oracle since they own it).
