We have about 30 servers running Debian for 800 workstations and for about 50,000 Internet users by day. The standard configuration is used in Debian, running software as qmail, djbdns, vsftpd, Apache and Tomcat. We develop applications and web services in that architecture.
Debian means stability, easy configuration and a limited investment in hardware. The latter is not the most important item, because we buy new hardware for Debian, but some systems (even laptops) based on hardware from the year 2000 run without problems. Performance is very important, but it is not only a fix for Debian, it's also a fix for the software you install on it.
We also use Debian because is an company-independent Linux distribution which has a massive support community.