I would argue that FB is actually a mobile app with a desktop site available as a checkbox that was reduced to being just a copy of the app in React. It didn't

I would argue that FB is actually a mobile app with a desktop site available as a checkbox that was reduced to being just a copy of the app in React. It didn't really explode until after the iPhone and like 85% of their users are on mobile Apps. That said, an app for learning as an example should actually have a very different mobile experience as a user wont want hour-long study sessions on the subway (or in line at a subway restaurant.)