Duolingo has nothing to do with self improvement, it's just a shiny presentation for highly addictive game.
We should reward quality of engagement, not just length of it.
"Quality of engagement" is as hard to measure as "length of engagement" is easy.
People love to hate Duolingo, but I actually learned things on it getting real results. And it showed me that language learning does not have to be massive suck.
All in all, it is actually one app that is beneficial or at least harmless.