Are any large-scale services based on NixOS or Guix System?

Following the permeation of virtualization and containerization through world of network services, various tools and systems have emerged for provisioning and orchestrating such systems from templates with applied custom overlays. However, most such systems operating from a POSIX base have simply adapted standard Linux-based operating systems for the new environments. In contrast, [NixOS](https://en.wikipedia.org/wiki/NixOS) and [Guix System](https://en.wikipedia.org/wiki/GNU_Guix_System) would appear to be ideally suited for such applications, by their native architecture, centered around a read-only file base with overlays managed through a declarative language. Such design would appear to carry directly in the operating system much of the functionality intended to be injected into a standard operating system by projects such as Puppet, Chef, and Ansible. Is NixOS or Guix System currently in use by any large-scale services?