My response is WHY? WHY do people insist on having a "Dislike" button?
Last year I got into a discussion on Facebook with a guy I knew in High School about this same subject. He had posted the same copy/paste that I had seen going around at the time. It was something like "Post this in your status if you want a Dislike button, we need 1,000,000 people to make this happen" so I responded asking why. Why did he feel the need to express his dislike of something someone posted when he could simply ignore it and move on. Ignoring something by NOT clicking "Like" or commenting on it is effectively a "Dislike" since you were so indifferent to it you chose to not "Like" it. He argued that ignoring it wasn't the same because he wanted to be able to express his dislike of what some people post. Eventually he quit answering me after I kept pointing out flaws in his logic and shooting holes in his hate veiled as an excuse. Put simply, he wanted a method to express his anger, hatred, or contempt for stuff that people post but he didn't want to go to the effort to actually comment or talk to the person. Pretty pathetic huh?
Now, as Geek Sugar points out, the point of her post is not for a general "Dislike" but more of a situation where you don't want to "Like" your friends post about their cat dying but you want a simple one click solution to show your dislike of the situation showing your sympathy for them. This idea is a noble one and one that would normally be ok but lets face it, 99.99999% of people that would use the "Dislike" button are not so noble. They want a "Dislike" button so they can express their opinion and disapproval of your post. Like the guy from high school, these same people lack the courage to post a comment and enter into a discussion on whatever the subject is that they "Dislike" or they'd already be doing that. They want a simple, anonymous, button to click to push their opinion on you in an effort to belittle your post or establish their self-perceived superiority over your post/opinion.
A "Dislike" button would lead to nothing but abuse by those without the strength to own up to their own opinions. If someone keeps posting shit you don't like, unfriend them or hide their updates. Stop asking for a tool to try and oppress peoples updates and opinions.