user behavior - Why is the "dislike" button going into extinction?

I've noticed that Facebook and now Google+ only have a "Like" (or +1) button, in spite of users wanting a "Dislike" button. This seems to be a bit of trend. Disqus also dropped their dislike vote - now you can only "like" stuff.

Is there any particular reason for that (other than may be having one less button and less functionality to worry about)? This always struck me as a bad idea because it's harder to identify bad content (eg: stuff that would get massively downvoted but can garner some upvotes).

Answer

Down votes are useful when looking at a narrow interest community (like this forum). It is likely that one person down voting an item gives useful information for most of the other users. But if you're talking about a narrow topic in a broad audience, that down vote tells me very little.

Take music as an example. If you love classical music, does the fact that a lot of classical music haters down voted a great classical piece tell you anything? Not at all. Does it tell you something if it has a high number of up votes? Yes.

So for things like facebook, where there is a large and diverse audience, only having an up vote makes sense. People that don't like something simply don't vote on it.

Additionally down votes are useful when trying to rank or sort content. That is another reason why it is good on Stack Exchange for answers, but it also explains why it is not used on comments - we don't sort comments.