Steam has been setting and shattering concurrent user records over the last week, peaking at 8.51 million on January 4, 2015.
According to a report from VG247, there were 8.36 million users on Steam on December 28 - a new high for Valve's distribution platform as it enjoyed the holiday sales rush. But it wasn't to last.
On New Year's Day, Steam's concurrent users hit 8.47 million users, but even that was exceeded on January 4 when the figure reached 8.512 million. With the seasonal sales winding down, that figure seems likely to remain the high-point for the foreseeable future.
The most played game on Steam is Dota 2, and by a comfortable margin - more than 835,000 concurrent players at its peak today, versus 398,000 players for Counter-Strike: Global Offensive. Dota 2 was the most played game back in June 2014, when Steam reached 8 million concurrent users for the first time.