Counter-Strike: Global Offensive is over ten years old, and much of the game design that underpins it dates back to the original mod release in 2000. Yet it's arguably more popular today than ever, after breaking its all-time concurrent player record earlier today.
Earlier on February 11th, 1,320,219 people were playing CS:GO simultaneously according to Steam's own stats. That beats the previous record of 1,308,963 from April 2020. You can see the historic data most easily via SteamDB.
I think it would be reasonable to speculate that the peak in April 2020 was prompted in part by a global pandemic increasing player counts across gaming in general. The latest peak is harder to diagnose, although there was an update yesterday which introduced new community-made weapon finished and stickers. Perhaps the final push was a lot of people wanting to get Denzel Curry's music in-game.
The more likely reason is just that CS:GO has been steadily increasing in popularity for months, with previous recent weekends already getting close to breaking the record.
As noted above, in a genre of near constant change, Counter-Strike has remained remarkably static for nearly two decades. CS:GO's innovations came via new competitive ranking, new progression systems, and a handful of new maps, while its modes and arsenal of weapons went mostly untouched. Valve did make a rare tweak to two of its most popular weapons last November.
While CS:GO is obviously in rude health, Team Fortress 2's community have been protesting in recent months due to a lack of support from Valve. Valve announced this past week that they would release a major update for their other team shooter this summer, although the announcement post mostly consists of Valve
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