Not entirely, it's about 50% Windoze, 50% Linux/Unix. 90% of the servers run Unix or Linux, Windoze servers are more likely to be hacked. Since most businesses now use servers for all networking this makes Unix and Linux just as common. Now, why is Windoze more likely to be hacked? Simple, most hackers learned on machines with the Winidoze GUI and MS-DOS (the really good ones anyway) and Linux and Unix have better security measures in place that are integrated into the OS better. Windoze was not originally designed for security, and therefore most of the security programs are utilities and not part of the core code, making it easier to bypass them or if they are in place well enough they are annoying to the normal user. This makes Linux an Unix very useful for networks, as well as the added advantage of a ton of open source utilities and tools to choose from, so no matter what hardware you have there is something that will work in Linux and Unix. Mac bypasses all that by making their hardware and software dependent on each other.
