I think Duolingo offers a wonderful service, and I really enjoyed it while I was using it. Currently, my only feedback for the team (if they're reading this) is

I think Duolingo offers a wonderful service, and I really enjoyed it while I was using it. Currently, my only feedback for the team (if they're reading this) is that it might be a great idea to partner with popular exchange programs to notify highly-active users of opportunities to live in another country for a while to hone their skills even more! For example, I participated in Explore, a Canadian immersion program that offers a chance to live in Québec for five weeks and really improve your French by speaking it day-in and day-out. Targeting such programs to their users could be a great value add.