This situation is rapidly changing too with ATI and intels OSS GPU drivers.
Intel drivers are Ok though.
When I open Steam it opens in the Store view in the main window, and the promotions pop-up also appears with sales and promo info. Great, except that 90% of the time the main store page and the pop-up are pushing Windows only games. I game on an iMac and occasionally on a Linux laptop. I have never bought a Windows only game. I send survey info to Valve.
If Linux (and Mac) gaming is to grow Valve need to try harder to get the games into my field of vision. It's silly that I click on the details button of the promo pop-up for a whole series of games only to scroll through the list and mutter "windoes only" to myself repeatedly. It simply is not content aimed at me at all, and they know who I am and the platform I am using.
http://phoronix.com/scan.php?page=news_item&px=Hwsurvey-Febr...
Even if we generously agree that it's correct, how much has steam grown? A 0.05% drop could still be more total users than the previous month.