Why can't Facebook shut down the scammers?

Why is it so hard for Facebook to remove these ads to what are obviously scam sites? What does it take?

Since publishing stories on the issue several readers have said they reported the ads to the social networking site but they continue to appear. Even after my inquiries and stories, the fraudsters are still being permitted to advertise their wares.

Have you been tricked by one of these scams promoted on Facebook? Is it just a case of buyer beware or does Facebook have a responsibility to be more vigilant.