Per the Steam hardware survey, the Ubuntu Linux usage currently comes down to:
Ubuntu 12.10 64 bit - 0.29%
Ubuntu 12.04.1 LTS 64 bit - 0.26%
Ubuntu 12.04.1 LTS - 0.13%
Ubuntu 12.10 - 0.12%
So all in all, the Steam Linux usage is just at about 1% with Windows platforms dominating about 95% of this gaming platform, but at least the 64-bit Ubuntu Linux builds have much greater popularity than their 32-bit counterparts.