To me this looks a bit like a "charity" investment. I've heard rumours Duolingo was running thin on capital while frantically searching for a business model.

However, despite not being profitable yet, Duolingo is doing a tremendous amount of good. I know quite a bit about language learning and learning in general, and I have never seen a better tool for learning the languages they offer. The combination of words, sentences, smart repetition and text-to-speech is pretty much unique, and the courses I've seen are quite well designed.

Caveat: No, Duolingo alone is not enough to get fluent in a language, but if you used it as your only (or major) tool, you'll get very close.