Why social media is important to the marketer:

In the mid-1990s just about everyone used direct mail and fax blasts to market on a mass level.  Then fax blasts became a problem because people complained that the “blaster” was using up their fax machine ink and paper.  So marketing moved to email and email “spamming.” 

Then there came the “CAN-SPAM” law which set the rules for commercial email and established requirements for commercial messages.  Today, social media is minimally regulated and rather than utilizing a one-to-many one-way communication it encourages and facilitates one-to-many or many-to-many two-way communication. 
It’s about the sharing of information and feedback and personal opinion and commentary.  It more closely reflects what happens in the real marketplace where people have conversations, rather than simply receiving one-way marketing communications.