Fastly largely runs on Varnish, it seems: https://www.fastly.com/blog/benefits-using-varnish

>At the core of Fastly is Varnish, an open source web accelerator that’s designed for high-performance content delivery. Varnish is the key to being able to accelerate dynamic content, APIs, and logic at the edge.

I think Fastly is the one having problems (they happen to use varnish but I haven't seen anything which says varnish is the root cause) - so all sites using it are down.