The main item that's been interesting to monitor with the Steam Hardware Survey has been the Linux usage metric. While Linux gamers made up around 2% of Steam's customer base, in recent months the number has been dropping.
It's difficult to determine a precise number for the Linux market-share on Steam since the numbers are broken down by distribution and there's an "Other" category. However, for May 2013 it appears there's no significant change in the Linux usage. Linux gamers still appear to make up just over 1%.
Ubuntu Linux use had increased for Steam by almost two tenths of a percent but overall the Steam Linux usage appears just over 1.3% in total or closer to 2% if possible counting some of the "Other" users.
The May 2013 Steam Hardware Survey can be analyzed at SteamPowered.com.