It's easy to see why Fortnite would grow so fast, not just because it is free-to-play. In addition to your presence in the cultural zeitgeist (you know you are mainstream when Big Bang Theory, Saturday night live and World Cup players refer to it), it is simply available on many other devices than it was in early 2018. Between the launch of iOS in April, the arrival of the Switch version in June and the gradual beta in recent months, it would be almost harder to find a place where you can not play one or two rounds.
For Epic, the question is not only whether it can continue to grow its user base, but whether it can sustain that interest, as growth slows. You're trying, at least. There has been a steady flow of limited time modes to increase gameplay, and the company has dumped $ 100 million in e-sports tournaments. Developers know that they can not afford to rest on their laurels, and are willing to spend a lot of money if it brings people back.