I bet if you are an active facebook user then you might have already seen such post. They lurk user into opening the link and then a background script will automatically tag all the victim friends to that same video or it can also prompt you to install malicious software in order to view the same. If you are not logged in then it will prompt you to login like shown below: So better next time before opening any link or installing any unknown software be sure whether you trust the content originator fully or not. As an advice always open external links in incognito mode of same browser or different browser in which you are not logged in.
Security Beyond Compare ..