Newsbrief: Steam Spy, a side project launched by programmer Sergey Galyonkin that tracks a variety of Steam statistics, has revealed that since the start of 2015, users on Valve's Steam platform have spent over half of their combined playtime on Valve releases.
Valve's dominance stems from the success of internally-produced titles such as DOTA 2, Counterstrike: Global Offensive, and Team Fortress 2, which top Steam's player charts on a daily basis.