Duolingo seems to have removed the "Following" tab from the Forum. Here's a link to it.
Edit: It has returned. Rejoice!
It looks like new discussions are not added anymore since today 3 October 2017.
For example, if you click on that link do you see that this discussion (which you thus created) is added to your list of followed discussions?
Very probably not.
You will very probably only see a list of older discussions.
I can see it again now. :)