Windows - 40.8% Unix - 29.7 Linux - 14% z/OS - 9.4% Where's OSX??? Worldwide Server Market Decelerates in Third Quarter, Exhibiting Significant Signs of Weakness for Fourth Quarter and Beyond, According to IDC
Interesting that they segregate out Unix and Linux, in fact, these statistics are very suspect. They only take into account servers shipping with an OS. Most deployed servers are in fact running linux when the numbers are computed by market share. Most linux installations are not enterprise level like these statistics are tracking.
Are you talking about the servers on the internet? http://news.netcraft.com/ Then you would be right that Linux is deployed the most. However, I don't think "web servers" are the majority of the servers out in the world. You don't need that many internet servers to have a web site. You need like one most of the time, and maybe a few backup...and you can have this hosted too with like lots of traffic. Most servers are only internally accessible (or externally by privileges) and not public. Also, note that IIS is not just a "web server software". It's really an Applications Server that can be used for browsers (and consequently the internet as well). Then you'll have to add Windows Server 2003,2008...and I'm not sure if they count SharePoint and Exchange and stuff like this here.