I just ran across this great talk at last year's Web 2.0 conference by Tara Hunt on a knowledge management blog that I frequently visit.
She talks about how the leveraging of the feeling of reciprocity and social capital and the nurturing of a gift economy in the design of your product or web site's user experience is the most efficacious way to promote your message. She calls it The Whuffie Factor which is a term inspired from a book written by Cory Doctorow called Down and Out in the Magic Kingdom.
She also contrasts this approach to traditional PR and shows how The Whuffie Factor is a logical extension to the prosumer approach of establishing authentic conversations.
The Whuffie Factor: The 5 Keys for Maxing Social Capital and Winning with Online Communities (Tara Hunt) from Steffan Antonas on Vimeo.