The most Nix-centric companies are all consultancies and startups, but off the top of my head:

  - Target
  - Shopify
  - Pinterest

all used Nix fairly heavily at some point, and I believe they still do.

(This is actually about more than NixOS— the binary caches affect all Nix users, including those on macOS as well as any Linux distro.)

Shopify and Target have been known to use it.

repl.it comes to mind, but I think recently a few others have popped up.