Facebook appears to have quietly added up/down voting to comments

I'm seeing it. This is different from a "dislike" button though.

When you have blog posts and other content with hundreds or thousands of comments, chronology is no longer an effective way of sorting. This idea seems to make sense. I think that I heard that YouTube finally did something similar with video comments.

I saw this the other day, I think it is ONLY on their blog.

What is the source of this screenshot?

http://blog.facebook.com/blog.php?post=2356432130

You might be in an A/B test.

He might be. I dug around and can't see any such buttons in my account.