Steam hosted $3.5 billion in paid game sales last year

SteamSpy also noted a declining rate of new releases across 2015

Steam was the venue for more than $3.5 billion in paid game sales, according to an analysis of 2015 from SteamSpy's Sergey Galyonkin.

SteamSpy's methods are based on games activated on Steam, so a portion of that $3.5 billion came from products purchased from other sources. However, while Valve may not earn revenue from every cent, Steam is still the hub for that much business. It is also limited to paid sales, so it excludes money that flows through Steam's growing crop of free-to-play games.