Valve has updated their Steam Hardware Survey that also provides statistics on used Windows/OSX/Linux versions for this digital game distribution client. It appears that the Steam Linux usage during August dropped and the overall Linux count is below 1%.
Head on over to the Steam Survey
to view the results in full, but they aren't too great for Linux users and are certainly down from when Steam debuted on Linux last year.
For August 2013, the Ubuntu 13.04/12.04 numbers all dropped and in the best case the "Linux 64-bit" report maintained its current count at 0.10%.