Excellent article about Duolingo . I used Duolingo for quite some time, and never had any progress. But I liked it anyway. Thanks for explaining why we didn't make any progress (without bashing the app too much).
One thing that the article doesn't mention is, that Duolingo does not get the spaced repetition right. At least that was my impression.
I wonder if a similar article about Babel could be written? I found Babel much better to learn something. Still, the progress much slower than learning with a book.