According to Steam’s latest hardware survey results, Windows 10 is already on nearly 2.21% of Steam users. Windows 10 nearly doubled the market share given the fact that RTM was released only on July 29th. In comparison, Windows 10 is now more popular than any individual OS X version. My bet is that Windows 10 usage among Steam users would have crossed combined Mac OS user share by now. But, we should wait till next month to see the actual numbers.
Windows 10 has been installed now on 18.5 million machines worldwide. Read about it here.