Activision lost 50 million users in one year and it's possible Call of Duty Vanguard is a main culprit.

Activision-Blizzard's latest earnings report shows a significant decline in earnings and monthly active users. The Activision segment, which serves up Call of Duty games every year, was hit hardest by the player drop.
According to data compiled by us and provided by the publisher's SEC filings, Activision lost 50 million MAUs from Q1 2021 through Q1 2022. This represents a heavy 33% drop in monthly active users. While Activision doesn't mention the reasoning for the player drop, it does say that Vanguard's lower sales and reduced Warzone engagement were key drivers for net bookings drops.

It's likely that Vanguard and Warzone are the reasons for the substantial player drop, but it's not as simple as both games doing poorly. It's more apt to say Activision's games simply aren't doing as well now as they were in Q1 2021.
Q1 2021 was a big period for Activision. The company recorded 150 million MAUs driven by Black Ops Cold War's popularity, new Warzone content, and the Tony Hawk Pro Skater remaster that launched in the tail-end of the quarter.
Fast-forward to Q2 2022 and we see Vanguard's lower-than-expected sales impacting MAUs as well as steep competition from other games like Fortnite and Roblox, both of which are heavy-hitters for MAUs.
To be clear, Activision is still no slouch when it comes to users. 100 million MAUs is incredibly successful even if it doesn't beat the company's previous high watermarks set in Q4 2019 (Modern Warfare), Q4 2020 (Black Ops Cold War) and Q1 2021 (Tony Hawk + Black Ops + Warzone).
Still, though, it's important to note the steep decline in MAUs throughout this period.