I tried Duolingo for French and Russian, and dropped both after a week.

Part of it is that I have nothing invested in either language and was just curious, but I also think Duolingo itself is a bit flawed; it repeats way too much material and makes you jump through too many hoops. I was busy and it was a hassle.

I think if you're going to study a language, a textbook + Anki is the way to go. If you tell Anki you know something, the algorithm believes you and shows it to you less often so you have more time to focus on what matters. It doesn't make you translate "the black cat" five times in one sitting.