Valve has released new Steam user survey data for March 2013. Interestingly though, the Linux usage hasn't risen but rather appears to be stagnant or on a decline.
Linux distributions now account for about 1.7% of those using Valve's Steam client. The numbers in this official survey are down slightly from last month's ~2% Linux usage
though not too much of a cause for concern. I don't expect the Linux usage to grow significantly until the Steam Box console arrives while the Linux desktop usage will be in the single digits.
The March 2013 survey results in full can be found at SteamPowered.com