The bulk of our SME backbone infrastructure runs on Debian.
We chose Debian back in 2004 as our preferred option after an extensive review process and considerable experience with (inter alia) Red Hat, SuSE, Ubuntu and Slackware. We have recently rerun the process and have again chosen Debian as our prime infrastructure tool. We prefer Debian for its package management system, its security management, its strict adherence to policies, and its relatively slow upgrade cycle.
We use Debian as a KVM host, to provide NAS/NFS/iSCSI services, SMB, for archiving servers, for monitoring/NOC hosting, web serving, email, and many application servers. We currently run two small private clouds based on Debian. We have just under 100 instances in total at present.