5 sources

http://www.msimail.net/why-is-facebook-so-popular/

 

http://www.lifestoryandme.com/2011/06/why-facebook-is-so-popular-psychology.html

http://mashable.com/2006/08/25/facebook-profile/

http://socialnetworking.lovetoknow.com/How_Did_Facebook_Originate

 

http://blog.thoughtpick.com/2009/10/the-addicitve-psychology-behind-facebooks-use-popularity.html