Tac Anderson put together a nice article about the history of social media, starting with its beginnings in the Cluetrain Manifesto:

At first there was no terminology to describe what was happening. One of the earliest attempts to put a voice to this, The Cluetrain Manifesto,  tried to explain the shift, years before the bubble popped, by pointing out that markets had become conversations. Companies could no longer push messaging at customers and expect them to act like sheep. 

Check out the full article.