How many people have actually continuously utilized Duolingo?
I know a lot of people use Duolingo, but how many actually try to learn more after they have finished a tree.
I have finished the French and Spanish trees and continue to use Duolingo to practice. I focus mostly on French which is the language I'm actively learning.
I did so much practicing and strengthening on my Spanish tree in the past that now the gold never fades. I still strengthen it from time to time, but I've waited up to two weeks in between strengthening to see if any bars would fade. They never did. There must be a formula so that every time you successfully strengthen without mistakes that the strength lasts longer every time. It's only recently that I completed the French tree, so parts of it are still fading every day.
I don't have any data, but it's likely that anyone serious enough about a language to finish a tree will probably continue to learn it in many cases. That might include Duolingo but there are also lots of other ways to learn too. Actually talking to people being just one example.