We use Debian throughout our whole self-managed infrastructure, including application servers (Ruby on Rails), frontend servers (using HAProxy, Varnish and nginx), database servers (using MariaDB, MongoDB and Elasticsearch), virtualization servers (using Ganeti with KVM), virtual machines and routers (using BIRD, keepalived, pimd etc.) for a total of almost 400 systems. Additionally most of our 120 staff use Debian on their workstations and laptops on a daily basis.
We chose Debian over the competition because of software availability, high quality standards, stability, comprehensive security support and reasonable release cycle.
For more in-depth analysis on how we use Debian, please have a look at the relevant DebConf16 presentation: https://debconf16.debconf.org/talks/44/.