Sergey Galyonkin's Steam Spy (opens in new tab), which keeps an eye on all sorts of Steam stats, comes out with fascinating titbits, like how refunds might not have hurt Arkham Knight (opens in new tab) that much, or that the average discount in this year's winter sale is lower than it was last time. Or, as the latest analysis (opens in new tab) tells it, that the paid game market on Steam was worth $3.5 billion last year.
Steam Spy estimates ownership of games—the figures don't come direct from developers—and it can't distinguish games bought elsewhere and activated on Steam from those bought via the Steam store. There's clear margin for error, but what's a hundred million dollars between friends?
In 2015, 3,000 new games found their way onto steam, up from 1,900 the year before, and 350 million games were sold. More impressive still, these numbers don't include revenue from free-to-play games or DLC. When you include purchases for Dota (opens in new tab) and Team Fortress 2, Galyonkin estimates that Valve controls 15% of the global PC games market.
Interestingly, it seems the tide of games sweeping on Steam has stopped accelerating (or at least is enjoying a rest before swallowing all that we hold dear (opens in new tab)).
The full analysis is exhaustive, breaking down Steam sales and the biggest games of each month of 2015. If you've got a head for figures, it's a good opportunity to ogle Valve's monstrous finances.