Some of the discussion here may be PART the reason...but only a small part. If you really think about what Facebook and Twitter are trying to do, they're trying

Some of the discussion here may be PART the reason...but only a small part. If you really think about what Facebook and Twitter are trying to do, they're trying to construct 'graphs' -- a social graph or an interest graph, depending upon which suits your cup of tea; and graphs are based upon connections. When you "like" or "retweet" or "follow" something/someone, you establish and create a link in this gigantic web of connections. Think of it like developing a system of telephone wires. Therefore, in this model, it doesn't REALLY make sense to have a dislike or a 'dis-retweet' option because you can't create a 'non-connection' more than leaving a void space between you and said other object. That's why these words aren't in the vocabularies of these tools mentioned. At least that's the visual mental model that I have, and I would encourage others to consider this.