Estimates of between 40-75% of all servers are Windows based. While Linux is ahead in Web servers (71% market share), they aren't the only type of server going around...
These stats are hard to measure as they can't really account for people who just install free linux distros, but in terms of sold Linux based licenses, Microsoft is ahead. At the very least it shows the that the assertion "no one uses Windows as a server OS" is far from the truth. 5-6 billion dollars revenue a quarter is hardly 'no one'.