There is a group that does studies on different apps. They look at how much time people spend using a language-learning app and how much each minute of studying adds to a standardized language test. Turns out, Rosetta Stone and LingQ are the most efficient per minute spent in those apps. If I remember correctly, both are privately held companies, which I see as a better sign than Duolingo’s public listing. I don’t know about their privacy policies, though.