The first Fortnite was launched in July 2017, but the new battle royale feature was launched in September 2017. And Epic Games first released data on the number of players in January, when the number of downloaders reached 45 million times.
Then Tencent who is an Epic Games investor also confirmed that the game reaps 40 million players every month.
This is hardly surprising, because Fortnite's popularity is very high, and is available on many platforms. And the most important is, Fortnite is a free to play the game, which became one of the determinants of the popularity of the game.
The most recent platform that gets Fortnite is the Nintendo Switch, and now the only popular platform that has not missed the game is Android. But Epic Games itself has confirmed that Android will also be going to Fornite.