Mendix is a Rapid App Development Platform. The complete Mendix hosting platform is running on Debian, both VMs and hypervisors, currently (April 2015) about 3600 installs. The majority of these nodes run Mendix Apps (java) and PostgreSQL databases. But also these services run on top of Debian: backup servers (rsync, btrfs snapshots), routers (bird), webservers (nginx), loadbalancers (keepalived), vpn (openvpn, ipsec-tools/racoon), nameservers (bind), mailservers (postfix), vcs servers (git/svn), monitoring servers (nagios/munin). And, of course, the people who manage these nodes also run Debian on their desktop.
We are an incredibly happy Debian user (and contributor) because it is rock solid and we love free and open-source software.