It's really damning that nowhere in that post does it mention improving the learning experience. Gamification, league boards, notifications etc. - but nothing about improving the service so users learn the languages better.

I spent 6 months or so doing the Finnish language (to completion) on Duolingo, and although it was good for practicing Finnish verb conjugations - the lack of a decent grammar tutorial means that you have to learn that elsewhere, which all start from scratch.

A decent set of books (Suomen Mestari in my case) and Anki are far more powerful for learning.