What is the little up and down arrows with the numbers by it on the discussions in the bottom left corner?
It is away of showing if you like or dislike a post. The up arrow says you like the post, the down arrow says you dislike the post.
PS posts receive 5 or more down votes the are hidden from view in the forum. It is also possible that moderators can delete these posts if they feel that they go against Duolingo's guidelines.