You have full control over the comments that appear on the video page for videos that you upload to YouTube. If you decide for some reason you want to remove any of the comments left by other users, ...

It's been there. The caveat is that you can't delete a comment after someone places a more recent reply in the thread. After that you can only edit, and the post will be flagged to reflect that.

Facebook comments can disappear for one of two reasons: the person who posted the comment on a post deleted it or the person who owns the original post deleted the comment because they didn't like it.