I'm technically working on a competitor to Duolingo (for Japanese), so I'm definitely biased, but personally I'm a little sad with how much success they're having considering how bad their system is (for Japanese, can't comment on other languages, but I can't imagine it's a lot better?).
There are a lot of considerably better options for learning Japanese (and I'm not even talking about my own app as it's still a huge work-in-progress), and yet for some reason I see posts from people using Duolingo on forums everyday. It's really popular.
I make a point of trying out most of the apps/websites in this space (since it's always a good idea to see what others are doing better than you), and (to me) it's apparent that Duolingo is definitely on the bottom of the barrel when it comes to actually being an effective learning tool. It might be okay-ish for a total, 100% beginner, but it breaks down very very fast.
I guess you don't need to have a good product as long as you have solid marketing, flashy UI and heavily gamify the process? That was somewhat eye opening for me.