I suspect that many Duolingo users are not choosing between Duolingo and a formal language course. I suspect they're choosing between Duolingo and some other smartphone activity.
I enjoyed my formal language education but four years of high school Spanish was about as useless as any other high school class. The ~10m a day I spend on Duolingo is as much of a "waste" of time in terms of language aquisition, but it's almost as enjoyable as my language classes were, feels slightly less wasteful than whatever other smartphone game I'd be playing, and lets me maintain the fantasy that one day I'll actually get around to (committing the time necessary to) learn a foreign language.
I enjoyed my formal language education but four years of high school Spanish was about as useless as any other high school class. The ~10m a day I spend on Duolingo is as much of a "waste" of time in terms of language aquisition, but it's almost as enjoyable as my language classes were, feels slightly less wasteful than whatever other smartphone game I'd be playing, and lets me maintain the fantasy that one day I'll actually get around to (committing the time necessary to) learn a foreign language.