Nice write up on how StackOverflow moved to HTTPS and how they utilise Cloudflare https://nickcraver.com/blog/2017/05/22/https-on-stack-overflow/
The way I read that article, they are not using Cloudflare:
Moving to Fastly was relatively recent, but since we’re on the CDN/Proxy topic I’ll cover it now. The move itself wasn’t terribly interesting because most of the pieces needed for any proxy were done in the Cloudflare era above. But of course everyone will ask: why did we move? While Cloudflare was very appealing in many regards - mainly, many data centers, stable bandwidth pricing, and included DNS - it wasn’t the best fit for us anymore. We needed a few things that Fastly simply did to fit us better: more flexibility at the edge, faster change propagation, and the ability to fully automate configuration pushes. That’s not to say Cloudflare is bad, it was just no longer the best fit for Stack Overflow.
(Quote taken from the article linked above; bold added.)
yeah they may have moved on, but still details how they did use cloudflare
Sometime ago I already read some of these articles and totally forgot about his blog. Thanks for reminding.