I have always been curious about how much business Steam gets. The release of FEAR 3 provides the opportunity for a rough estimate.
FEAR 3 has a steam achievement for reaching level 10.
The global achievement stat percentage for this achievement is 5.8%.
A quick skim through the ingame leaderboards shows that approximately 270 people have reached this point. There is some overlap between different boards, which I've somewhat adjusted for.
The #1 selling game on Steam has sold less than 5k copies/$250k. That's not a lot for a solid-B release. In fact, that's an upper bound. People that got their copy via retail or another digital distributor also count towards the 5k player estimate, as do preorders.