Brand awareness and the power of recommendation are old marketing concepts. However, with the introduction of social media sites, it has evolved to a whole new level.
(Marketing Week: http://www.marketingweek.co.uk/analysis/essential-reads/the-new-word-of-mouth/4003281.article)
Even without monetary incentive, people advocate or criticise products, and share information; it’s a social element.
(Small business marketing Forum: http://sbmarketingforum.wordpress.com/2014/04/11/how-does-word-of-mouth-really-affect-my-small-business/)
People can become unofficial brand advocates.