As web applications are more and more packed with information, the need to hide controls have emerged. The option would be to have even longer web pages, showing a lot of redundant controls for every post as in "unfollow post, unfollow updates from user X, unlike page, still like page but don't show updates, and on and on and on.
This has made designers hide buttons which you can only access on hover. Sometimes these are very long sequences on hover leading to hover leading to hover which if you eventually miss a spot, loose all the sequence in once. You have to start over.
This leads to great cognitive load on the user, which is very bad usability. So in general - showing buttons on hover, especially in sequence, is a bad thing.