People are happy to talk about companies and products they like, and millions of people do it for free every day, without prompting. BU as soon as you offer to pay people to refer other customers, any interest they had in doing it for free will disappear.

Customers’ decisions to share or not will no longer be based on how much they like a product or service. Instead, the quality and quantity of buzz will be proportional to the money they receive.

-Contagious, p.59