After looking around the internet for quite sometime i have found a very useful article about the history of social network sites.
http://jcmc.indiana.edu/vol13/issue1/boyd.ellison.html
This article seems to be a piece that has been submitted for some sort of work, very text based and well referenced.
It goes through a definition of social networking, the early years of social networking, and then goes through a number of different popular websites and how they grew in popularity and then was replaced by another.

Timeline from article