Even if Facebook never adds a "Dislike" button, the site at least understands that disapproval has its place.
Websites using Facebook's social plug-ins may now include voting systems in their comment sections, not unlike the up-and-down approvals in the comments on Digg and Reddit. Instead of "Like" and "Dislike," the voting system uses a green up arrow and red down arrow.
You can see it in action on Facebook's latest blog post, which talks about ways to stay secure. The vote count appears next to the person's name, and the arrows are on the right. Strangely, there doesn't seem to be much organization in the comments based on votes; liked and disliked comments are all over the place.
Keep in mind that the voting system doesn't apply to profiles on Facebook itself. It's just a Website plug-in, similar to the "Like" button you see above this post and many others across the Web. In that sense, the upgrade to Facebook-powered comments is an answer to existing third-party comment systems like IntenseDebate and Disqus.
That's the real story, even if the like/dislike debate is more intriguing. Facebook's hooks into the rest of the Internet are growing deeper. If the upgraded comment system attracts more Web sites, Facebook gets more data, not just on what people are reading and sharing, but what they're actually saying.