I think it can be used for actual language learning. But no, it's not going to do it by itself. Say each word or phrase out loud, multiple times, and possibly other "extra" study (whatever works for you), and the daily DL habit will certainly leave you with better vocabulary and hopefully more than what you start with.
If you just do a single lesson braindead as fast as possible so you don't lose your streak, then no, you won't learn a language.
I have to also say that none of the languages I've studied on DL are completely new to me. I have had formal instruction in each at some point in my life. I've noticed certain concepts that would probably not make as much sense to someone who hasn't studied the language or language in general (case, conjugation).
Finally, I have one major complaint about Chinese on DL, which is that after 9 months or so I still see the pinyin full time, which means I never really get a chance to solidify the characters properly. I've already said to myself that I need to practice writing separately. There are other problems, but overall, you're probably getting more out of 10 minutes a day of Duo Lingo than several hours of whatever other mobile timesink you're using.
If you just do a single lesson braindead as fast as possible so you don't lose your streak, then no, you won't learn a language.
I have to also say that none of the languages I've studied on DL are completely new to me. I have had formal instruction in each at some point in my life. I've noticed certain concepts that would probably not make as much sense to someone who hasn't studied the language or language in general (case, conjugation).
Finally, I have one major complaint about Chinese on DL, which is that after 9 months or so I still see the pinyin full time, which means I never really get a chance to solidify the characters properly. I've already said to myself that I need to practice writing separately. There are other problems, but overall, you're probably getting more out of 10 minutes a day of Duo Lingo than several hours of whatever other mobile timesink you're using.