One thing to keep in mind that their work-load is very ready-heavy which eases things a lot when scaling the system. The same is true for Wikipedia. Scaling a write-heavy workload is way more complex than scaling a read-heavy workload.
I guarantee you the bulk of traffic to SO is hitting a page and performing zero writes.
Yes it's read-heavy but there's still plenty of work done in assembling a page. It's definitely not as simple as caching at the CDN edge for every hit.