Newsbrief: Sergey Galyonkin, founder of Steam stat tracker Steam Spy, has revealed that, so far, only six Steam titles released in 2015 - GTA V, ARK Survival Evolved, H1Z1, Cities: Skylines, Rocket League, and Besiege - have sold over 1 million copies.
Aside from those, only six other titles have surpassed the 500,000 mark, with Dying Light, The Witcher 3, Stranded Deep, Killing Floor 2, Total War: Attila, and Pillars of Eternity each shifting over half a million copies on Valve's online marketplace.
Update: We clarified the article to show that the sales milestones refer to games released in 2015. This article does not include games released prior to 2015 that hit the same milestones (e.g. Counter-Strike GO sold 1 million-plus copies this year, but was released in 2012, thus it is not part of this set).