StackOverflow's database doesn't need to work that hard because the vast majority of traffic is read requests for public, essentially static webpages that get re-rendered and updated a handful of times in their lifecycle. I'd guess 90% or more could be served by a reverse proxy without ever touching SQL Server or even the app tier.

It would be easy to exceed its DB load with an application that serves unique content to each user, is write-heavy, etc.