Duolingo claims to help you learn a language for free because as you practice translating sentences you are simultaneously helping them translate the Web.
It is not clear to me whether translation is the only methodology behind their language learning approach, and it is so not clear what exactly they mean by the Web to be translated (which websites) or how it is done (where the information is stored, who uses it, how are the best translations of the same sentence selected, where the sentences come from, etc). Relying solely on translation as a method to learn a language can be tricky and ineffective. I hope Duolingo offers more than flash cards and translation questions, otherwise, the only value proposition that makes it different from a flash card software (which there are free AND open source ones out there) is the fact that the students are also doing some greater good by simultaneously translating the Web).
I still need to see the product and play with or for a full review (which I will add as an update to this post as soon as I can access it), but from the video, it does not seem to offer a variety of valid and effective activities that employ different approaches for language learning.