Ask HN: Monorepos in the wild?

I've been reading up on monorepositories vs per-project repos and blog posts often namedrop google and facebook as places where everything is in a single, monolithic repository, but I didn't find any solid references to back that up. Anyone here working on a monster-scale, multi-project/multi-component product that is managed with a single repo? what are your experiences compared to per-component or per-project repos?