If you have a closer look at Steams hardware and software poll, then you'll notice, that Windows 8/8.1 have been able to crack the 20 percent barrier for the first time ever.
In December 2013 exactly 20.06 percent of all Steam users accessed the service using Windows 8 32 bit or 64 bit. Other than that it is rather interesting to see that Windows 8 still accounts for 11.22 percent, which makes it still more popular than Windows 8.1. Although it looks like it won't take long until Windows 8.1 is going to overtake Windows 8 in terms of market share. But as soon as you start looking at Windows 7 market share it becomes clear, that gaming is done, almost avoiding Windows 8. 63 percent of all gamers on Steam still use Windows 7 and they might stick with it for quite a long time still.
Source:
Steam