Don't you think that the words you learn don't circulate enough in the exercises?
I forgot most of the words that were supposed to be in my long term memory. Of course I can do the vocabulary exercises manually. But I thought Duolingo's thing was that it would show you the words you were least exposed to through the exercises, automatically.
I think that's why they added the vocabulary section, so that you can keep track of all the words you've learned. Of course, you're also welcome to revisit any lessons you feel you may have forgotten. Duolingo isn't perfect, so I try to combine it with other ways of learning French, cos this isn't enough on its own, at least for now.
I find it's very helpful to write down sentences that I have missed, or which I haven't been certain about. Just the writing of them seems to help me remember, plus I highlight the tricky bits--accents forgotten, gender reassigned, grammar mangled. Then I go back every once in awhile and read aloud some recent pages in my notebook. I also sometimes write down sentences from translations which I have managed to puzzle out, but which I could never recreate.