SO was hosted on a single rack last I checked, not a single box. At the time they had an MS SQL cluster.
Also, databases can easily see a ton of internal traffic. Think internal logistics/operations/analytics. Even a medium size company can have a huge amount of data, such as tracking every item purchased and sold for a retail chain.
They use multiple servers for redundancy, but they are using only 5-10% capacity per [1], so they say they could run on a single server given these numbers. Seems like they've since moved to the cloud though [2].
