Word of mouth is powerful because it is rich communication. The hearer is not distracted, as they may be if doing an online task. It is also powerful because it can turn your customers into salespeople. What is more powerful for Nike, 10 sales people or a million that rave to their friends? Word of mouth is much more powerful than anything online and much more powerful than great sales people.
There is a great book by Jonah Berger called ‘The Catalyst’ that breaks down the components of great viral and word of mouth implementations. There is a psychology of why we share things. The technology changes but the psychology of why we share has been the same for thousands of years. There is a science as to why people share something. We encourage you to read Jonah’s book.