And the fact that Reddit has literally dozens of apps across all of the mobile platforms. Having a mostly open API gave 3rd party developers the opportunity to create better interfaces to draw users into using Reddit.
When I first started using Reddit over 6 years ago, it was mostly desktop, but these days, probably 90% of my Reddit usage is mobile because I feel more productive (ironic) using a nicer interface with gesture support for voting and replying.
I would bet that there are a million+ users primarily on 3rd party mobile apps around the world who, if not more.
https://i.reddit.com is a good balance between app and the desktop website. A lot faster than m.reddit.com and supports the basic features of reading comments, commenting, and submitting. It also doesn't crash all the time and saves data.
Narwhal on iOS is pretty great. First-party reddit app is quite bad. Ironically reddit heavily pushes their official mobile app if you visit on mobile Safari.
That said, are these numbers reliable? And: Don't a lot of people visit FB via an app? I assume that's not counted here.