In March the Linux usage was at 1.6~1.7% and now for April it's down to 1.5~1.6% when factoring in all of the different Linux distributions and their diminutive stakes. This is a noticeable drop from around 2% Linux usage when the Steam client first premiered.
Hopefully as summer approaches there will be more Linux desktop users having time to fire up their Steam client. Likewise, with Left 4 Dead 2 on Linux being imminent along with Counter-Strike: Global Offensive on Linux hopefully in the coming months the Linux use will increase. Having a bigger impact will also be Valve's Linux-based Steam console in the coming months.
The Steam Hardware Survey can be viewed at SteamPowered.com.