I am noticing that Duolingo now has ads. Have I been just not seeing them this whole time?
Duolingo seems to want us to buy this thing called Duolingo Plus.
Its how they have decided to monetize the site. There was an announcement about it a while ago.
They must have been so small you didn't notice them.
Duolingo Plus allows you to download lessons so you can do them offline but it doesn't continue your streak and there are no ads. Also, Plus costs 9.99 dollars a month.
There should be more features for Plus, though that also ruins the image of Duolingo being free for language learning.