Technology

They also used volunteer labor in the past for some language courses: https://duolingo.fandom.com/wiki/Incubator/Contributors

And naturally turned it into a heavily monetized product at some point after (or during, I am not clear on the exact timeline from memory), as capitalists tend to do.

Also, their "health system" is plain anti-learning (punishing of mistakes) and clearly there to browbeat you into subscribing.

And this last part I will be honest is pretty heavily just my opinion on them and my experiences with language learning apps as a whole but: The only good thing they have going for them is marketing. One of the most overrated language learning apps there is, which is saying something because the competition is pretty dismal. There are numerous apps that try to imitate Duolingo in UI and design, like how MMOs imitated World of Warcraft, instead of doing their own thing. And the end result is they are worse for it and Duolingo sets a bad standard. What Duolingo did right was gamifying learning, which works well for some people, in some contexts (I am one of them). But how you go about gamifying learning could be very broad and it is criminally under-explored in favor of chasing Duolingo popularity; throwing achievement systems, leaderboards, and "baby's first app" style aesthetic design with a cutesy mascot at the problem, instead of thinking outside the box. Mascot designs that can range from actually cute to nauseatingly trying too hard, like someone got a bit confused about what cuteness is.