Playing second fiddle is Team Fortress 2 with 20.3 million. The top twenty games are dominated by Valve, although Skyrim and Civilization V squeezed in. There's another interesting stat.
According to Ars Technica's sampling of Steam, almost 37% of all owned games haven't been played by their owners. Of course 'hours played' was only introduced in 2009.
"The “owned but unplayed” phenomenon is an extremely widespread issue across Steam according to our data. It shows up most frequently in games included in bundles, offered at heavily reduced prices, or simply given away to players who have yet to actually play them. It’s not limited to those games, though; in fact, out of the roughly 781 million games registered to Steam accounts, our data shows only 493 million, or 63 percent, have been played even once," they .
It's mostly down to hearing about certain games and then grabbing them during a deal or bundle, but then going back to playing something else and mostly forgetting all about your purchase. "...primarily those that are often offered in bundles or as part of sales, are played by only a sliver of the registered owners on the service."
Ars Technica'smakes for interesting reading in this digital store age we live in. Valve said they had 75 million active users back in January. Undoubtedly Steam has revived the PC games market by helping everyone distribute their releases to a dedicated core PC audience. If only we got around to playing them all.