A top priority for any social networking site is engagement and interaction, and as the number of friends grow for their users, these sites are constantly developing tools that will target appropriate content to the user.
If I were to ask "how many of you have had a pop-up appear that claimed to 'fix a virus' or 'clean your computer' while searching for information on the internet?" I believe most people would raise their hand.
Scams have been around for many years where criminals see an easy opportunity to prey on the vulnerable. Unfortunately, since the dawn of the internet and email, opportunistic criminals have been more creative in their methods.