According to some extensive research by Ars Technica, 37% of the games that have been sold on Steam or registered to Steam accounts through other means have never been played.
Ars Technica's research is based on publicly available information rather than specialist chart tracker information -- Steam public profiles. Steam keeps track both of what games each of its users own, and how much they have been played in terms of hours. In the case of games with achievements, it becomes possible to analyze players' engagement with games in even more granular detail, though this was outside the scope of Ars' research in this instance.
In total, there are about 172 million Steam Community pages, though not all of these are actively used; some have been abandoned, some may have been created and then left without purchasing a game; some may be "alt" accounts for players who wish to have several different libraries or identities for whatever reason. It's obviously impractical to crawl all 172 million pages and determine the state of things, so Ars' sample makes use of 250,000 valid Steam Community profiles at any one time, covering three days' worth of data on a rolling basis. This gives a representative and random sample of the Steam Community's behavior, backed up by comparison with public and private sales data.
You can read more about the methodology in Ars' extensive report, but here are some of the most interesting findings.
First up, according to Ars' research, about 37% of the roughly 781 million games registered to all the Steam accounts in the world haven't been loaded up once. Contributing factors to this could include things such as promotions whereby developers give out Steam keys after the game has been available (and perhaps completed by the player) for a while, Humble Bundles and equivalents in which players purchase the bundle largely to get their hands on a single game for a low price, purchasing titles in Steam sales while they're cheap -- or simply purchasing games with the intention of playing them "someday." We've all done it.
The most-owned games on Steam are almost all Valve titles, with the free-to-play Dota 2 out in front of everything else by a considerable margin -- 25.93 estimated registered copies in Dota 2's case and 20.3 estimated registered copies in the case of Team Fortress 2. Also popular are the free Source engine tech demo Half-Life 2: Lost Coast (12.77m), multiplayer shooter Counter-Strike: Source (11.99m), Half-Life 2: Deathmatch (11.02m), Left 4 Dead 2 (10.71m, and interestingly, Left 4 Dead 1 doesn't seem to appear in the top 20) and the original Counter-Strike (9.78m). A little further down the list we start to see some non-Valve titles, most notably Sid Meier's Civilization V (5.84m) and Skyrim (5.94m) but it's clear that, on the whole, Steam has worked out rather well for Valve in particular. Not bad for a piece of software people really weren't sure about trusting when Half-Life 2 first came out.
In terms of the most-played games on Steam in terms of total number of hours, Dota 2 is again the runaway winner, with an estimated total playtime of nearly four billion hours between all players, and an average play time of nearly 150 hours per player. Conversely, the most popular game in terms of average number of hours played per player is Football Manager 2014, racking up an impressive average of 169.2 hours per player -- though this figure doesn't take into account active play time versus the many, many people who leave it running in the background while they work.
The report has a number of other interesting findings, but one thing's clear from it all: although Steam has an extensive catalogue of games that is growing by the day, the service is still very much driven by big hits that can keep their players sticking around for a long time -- and that's something in which Valve is still leading the pack.