Facebook says it's testing a downvote button. If someone selects the downvvote, under the test, the comment becomes hidden to the user. In addition, users are offered “Offensive”, “Misleading”, and “Off Topic” options.
“We are not testing a dislike button. We are exploring a feature for people to give us feedback about comments on public page posts. This is running for a small set of people in the U.S. only.”
