When somebody posts something on Facebook, you can inform that person that you "like" his/her post by clicking a button. However, if you change your mind and decide that you don't "like" it, you click "unlike" to stop liking it. But "unlike" is not really the opposite of "like." In other words, you initially "take pleasure in" said comment, but by undoing your action, you somehow decide that the comment is "dissimilar to" something.
That's unlike anything I've ever seen!