I was on Facebook the other day and clicked on an article that took me to a news site and I accidentally clicked on an Ad. It took me to a site and popped up a real looking warning that Microsoft Security Essentials had found malicious processes and I needed to click to get rid of them. This would have infected my machine with something. This is the problem with Ads on Facebook and any site. You never know where it is going to take you. If this happens to you. Don’t click on the warning. Go directly to Microsoft Security Essentials and see what it says. I have found a rash of Malware on my clients PCs and this is probably how they are unknowingly getting infected.
Be safe out there!