Facebook is rolling out a new feature that will allow users to edit their own comments on the site. Instead of deleting and reposting comments, you'll soon be able to simply edit them directly. When you do, Facebook will show an edit history to all users that will show the previous versions of your comment.
Facebook tells us the history feature is to ensure that "subsequent commenters or likers have the full context of the conversation." Basically, should a commenter completely change his or her comment (which is a fairly common form of trolling, truth-be-told), everybody else won't wonder why the comment thread seems nonsensical. The change has already begun rolling out and all users should see it "in the next couple days."