Steam Spy, the web tool that scrapes data together from Valve's own publicly available information, estimates Steam was responsible for 15 percent of global PC game sales in 2015. Excluding free-to-play games, that's $3.5 billion dollars.
Steam Spy is a clever implementation of the Steam Web API created by Sergey Galyonkin. It uses Steam's own data to extrapolate sales and ownership information for games with a three-day delay. Galyonkin has baked the numbers for the year and writes that it was "the best year for Steam yet."
"Over 3,000 new games," were put up for sale in 2015 Galyonkin said. That's compared to only 1900 in 2014.
"Indiepocalypse is cancelled, you can stop worrying now."
The biggest months for the service were in April, when several big titles, including Grand Theft Auto 5, came out all at once and December during the annual Steam holiday sale.
As far as overall revenue goes, GTA V and Fallout 4 lead the way with $161 million and $123 million respectively. For total games sold they also maintained the number one and two slot, with the third place going to Ark: Survival Evolved and fourth to Rocket League. H1Z1 pulled in at fifth.
Galyonkin also breaks down the numbers by month. There are a lot of caveats to this data, including how Steam Spy accounts for free weekend promotions and the like. It's all available in his post on Medium.