Here's a more recent source[1] (unfortunately poorly sourced, but it does have a timestamp for each item). It shows 200 million installs (11/11), but 74.2 million iOS installs (11/7), 11.5 million downloads on the switch (10/30).

Android usage is complex, because it's still only available officially to Samsung Galaxy devices, but the amount of players looks like it might be similar to iOS (there are usage numbers from the link, but for different times).

So, just from the total installs number and the somewhat recent iOS and Nintendo Switch numbers, we can put the maximum number of PC installs at around 114 million, before we account for Xbox (released mid 2017), PS4 (released mid-2017) and Android (Samsung Galaxy only but I think it may well be in the tens of millions anyway, especially since there are tutorials on how to get it on other devices).

I think that will leave us well within the territory where Steam and Fortnite have a largely overlapping PC user base.

> Steamcharts consistently surprises me at how few people play the popular games I like, so I'm beginning to think Steam isn't as big.

I think most people just don't game on PC as much anymore. Most PC games are available for consoles now too, and it's much cheaper to buy a new console every few years than to keep a gaming PC updated. My brother bought a keyboard and mouse for his PS4, and played Overwatch with that. Probably just as good an experience as the PC version for much cheaper total platform cost over time.

1: https://expandedramblings.com/index.php/fortnite-facts-and-s...