For instance, paste the URL of any Youtube video on a comment and submit it. After submitting, you will soon see the video’s preview thumbnail that starts playing the video upon clicking on it. Likewise, the links you share in the comments also give previews automatically. Simply paste a link in a comment form and submit it; you will see the preview of the page along with a thumbnail.
I have been wanting this feature since long and Facebook has finally given it to us. Thanks Facebook for allowing video embeds in comments.