Here is the link to the article: http://www.wired.com/wiredscience/2012/04/facebook-disease-friends/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+wired%2Findex+%28Wired%3A+Top+Stories%29
According to this article people have joined Facebook because of pressure from their peers. When getting an invitation from Facebook and someone sees their friend is using Facebook, they are more likely to sign up. I feel like this is similar to everyday life. Say you are doing a heads up vote, you do not want to be the odd person out, so you will go with the masses and what everyone else is doing.
