I think linux numbers are always a little lower for a few reasons. For example, we tend to dual-boot (or as frob mentioned, use WINE or a VM), and thus, often get counted as Windows sales, if the Steam survey comes up when we're in windows. Additionally, releases for linux are often considerably after the windows version, and often, many will opt to just buy the Windows version than wait for the linux release. We're still a small demographic, to be sure, but I don't think quite as small as the survey shows.
I think the linux community is starting to hold out and buy things when they're officially released for linux, as it's becoming more commonplace (borderlands the pre-sequel and civ5 come to mind as day one releases, and countless indie games), so I think as that becomes more common the numbers will rise. And, of course the release of Steam Machines is expected to boost things quite a bit, but we'll see.
It's been said often, I'm sure, but I think linux is finally coming into its own and may soon be a real competitor on the OS front, despite the current small numbers. I've personally wiped my windows drive years ago, as with the exception of a handful of games, my linux drives can suit any need I have (not to mention the pretty large benefits of a linux system).
easier to use: win, mac, linux
I think these days, that's changing rather rapidly, especially for people who are slightly computer-literate, like much of the gaming crowd. Installation for Ubuntu, for example, I find far easier, quicker and painless than Windows. It's all of a ten minute process to install the OS, the drivers come pre-installed, and there's none of the incessant updates for the next two weeks that Windows has. I think there's still a bit of a learning curve for new users, but that's mostly due to it just being a different OS. Installing Arch for example can be a bit of an adventure, but for a lot of other distributions it's pretty simple. I have my parents on Ubuntu, and they're pretty old and not terribly tech-savvy.
Anyhow, my two cents, user experience may vary and all that
edit* I also completely mean the next civ game, not civ5, which came out years afterwards (thanks to aspyr, those guys are great :)).