Ars Technica has done some digging around
For all its popularity, there are parts of Steam that are clouded in mystery, especially when it comes to how many digital copies of a game are sold. Kyle Orland over on Ars Technica has done some poking around Steam user profiles and with some statistical analysis, he's come up some interesting stats. Considering there's an estimated 781 million active profiles, it's disheartening to hear 37 percent of games on Steam are unplayed.
Now all of this information is based on estimates, so take it with a grain of salt, but Ars is backed up with sales comparisons to titles like Rust and DayZ whose creators tweeted sales figures that match up with projections. The most "owned" games are Valve's own DOTA 2 and Team Fortress 2 which is not a surprise considering they're just free to install. Other Valve titles like Ricochet and Day of Defeat have been installed by just over six million users but have been played by less than 10 percent of gamers. Whilst Valve's games dominate, dedicated players of titles like Football Manager 2014, Skyrim, and even Europa Unirvasalis IV are racking up the hours.
Whilst the 37 percent unplayed games stat is a downer, it doesn't surprise me, really; Steam sales have made buying cheap games in bulk an event that happens numerous times a year (who else is expecting an Easter sale this weekend?). Couple that with bundle deals that let you buy some games you want and some you don't in the same purchase, it's no surprise that there's plenty of games in my library that I have not even installed. It's really worth looking through all these graphs and charts as it really paints a picture of what games are really popular on Steam.