Facebook is the one thing the entire world (at least the western world) has in common. Everybody owns a Facebook account, and spends a lot of time there. whether this is a good thing or a thing that has negative aspects is for you to decide. I however, think that the use of Facebook is expanding rapidly, too rapidly.
750 million different people have an account, an that means this is a huge opportunity for advertisement, and companies know this. This connects to the post about advertisement in general (https://boolij.wordpress.com/2012/11/19/advertisement/), but when people access Facebook for several hours at a time, the revenue that surrounds the advertisement on the social media is tremendous.
Social Media however, is an addiction, or better said, can be an addiction. People are online for hours at the time, myself, sadly, not a exception, and the uses and possibilities for the exploitation are large, maybe even to large.