Facebook users who clicked on ads on the website may have been subject to malicious downloads. Trend Micro reported that ads affiliated with a Facebook application would take users to an advertisement page but quickly redirect them to a website hosting the malicious content.
The reason this slipped by Facebook was primarily because Facebook only checks the validity of a landing page. Therefore, if the landing page were to redirect to a malicious site, it could easily slip through the filters. I doubt Facebook will take too much action regarding this problem until users revolt. Since users are too busy complaining about UI changes, this probably won’t happen. Like most free websites, Facebook makes a large chunk of money through advertising. Therefore, they wouldn’t want to put more difficulty on advertisers trying to advertise on the social networking website.