Valve performs a hardware survey each month and the results are published online. The last hardware survey revealed all the Linux operating systems that used Steam, and not just the official one, Ubuntu.
The rate of adoption for Steam on Linux has been a mystery until now because Valve only published data about Ubuntu flavors. The latest hardware survey, for February, is now listing all the open source operating systems that have Steam installed.
So far, 44 open source operating system have been listed, although some of them are duplicates or have a general name that could be attributed to another distro already counted.
The most used open source operating system is Ubuntu 12.10 64-bit, followed by its 32-bit companion. Interestingly enough, Arch Linux can also be found on this long list, along with Linux Mint 13 Maya, Gentoo, Manjaro, and Zorin OS.