Historically, it has taken years for a new Windows OS to gain a major majority share, mostly as institutions put off the upgrade and some consumers decide that there's no real need to update. Steam users, however, tend to be amongs the earliest of adopters -- the power users that jump at the chance to try out any new beta.
Of course, Windows 7 still leads the pack in a major way with just over 50% of the reported user base, but Windows 8.1 saw a 1.88% jump in January to place it at 10.41% overall. Meanwhile, Windows XP and Vista dropped down further, now reaching just 5.96% and 2.84% of Steam's reported OS user base (respectively). There's still a long, long way to go before Win8 surpasses Win7 in terms of Steam users, but there's no question that the latest edition is making serious progress.