Codecademy, Mimo, Enki. I'm not going to act like I know a lot about other options, and you do say that fata hits on more advanced topics which these may not.

I do think you have a cool niche here that isn't being covered, maybe your landing page should include more information about the Duolingo-esque features? I just know that pulls me in a lot.

For the record, btw, id DID download your app and I am going to spend some time using it. Other commenters here are talking about how spaced repetition isn't what's important, which is true in the same sense that you can't just use Duolingo to learn a language. But that doesn't make it useless.