Just because people don't play games on them (apart from MUDs) doesn't mean they're not "mainstream."
I'm just pointing out how Microsoft has a tendency to "innovate" by adopting things that already exist.
I did find this, but it is old.
Industry-leading Platform for the Web (May 2001).
New figures taken from the Netcraft Web Server Survey indicate that the majority of the Top 50 websites, as ranked by Media Metrix http://www.jmm.com/press/mediaMetrixTop50.html are running Solaris, making it the industry-leading platform for Internet Solutions. 40 percent of these sites are now running Solaris OE vs. 26 percent for the closest competitor.