According to Steam’s updated Hardware and Software survey, it seems that Windows 7 64-bit is still in the lead with 34.31%, but based on how new Windows 10 is and how fast it has risen, it is not a stretch to think that it will be very soon that Windows 10 will overtake Windows 7 in terms of the number of Steam users running the platform.
In fact it was just recently that Microsoft made Windows 10 a “recommended” update, which means that users with automatic updates turned on will see Windows 10 begin the installation process itself upon downloading. The actual installation won’t begin without the user’s consent, but it will be there should users choose to go ahead with it.
Also given that the update is free until this July, there are probably plenty of users who will take advantage of that fact and install it, so don’t be surprised if Windows 10 were to rise in Steam market share even higher.