How sharing contents of websites can make your facebook hacked?

Some times when you are using a website and want to share some material like videos or images of it on your facebook or twitter wall or timeline, a log-in pop-up appears.It is a small card like template where you have to enter your facebook /twitter e-mail and password.It may look very professional.For example in case of facebook it will be of blue and white color.Never enter your information here because it can go to the owner of that website.If you still want to share the content then open your facebook/twitter on a new tab and log-in here.Then go to that site on other tab and share what you want because pop-up will not appear now.