They have about 7.5m visits per day to Stack Overflow. That's about 86 requests per second, which perhaps at peak is several multiples larger than the average. I wouldn't consider that a particularly gargantuan website. That sounds like traffic that you could service with a reasonable fleet of web servers and database fleet, given caching. Quite a lot of actions on the site can be applied with eventual consistency, I'd imagine.

http://stackexchange.com/sites?view=list#traffic

I don't mean to say that the system or the problem that it's solving is trivial - I am sure it is difficult to get right. As websites go, it might be large, but as systems go it's not particularly high traffic among systems that receive traffic from machines rather than humans. Imagine that you operate a data center, and you want to sample CPU, memory, etc. from your machines every minute. If you collect 50 samples per machine per minute, and you have 258 machines, you'll be handling 86 samples per second. Storing 86 samples per second into a time series database is probably considerably easier than SO's website rendering, but it goes to show that high-traffic or high-frequency systems are common in companies beyond small to medium size. It is easy for cross-cutting concerns like this to have massive request volumes, far greater than the human-generated traffic to any website.