Since I was curious it seems like they were valued at about 470 million in June 2015 (where it seems they had roughly 100 million users). I don't know how they plan to monetize (certified tests are mentioned and it seems they dropped their idea to crowd source translations) but it seems like they focus hard on staying free to grab even more users. Probably a decent case study for "defending VC" against bootstrapping.

TechCrunch article on their series D: http://techcrunch.com/2015/06/10/duolingo-raises-45-million-...