Word of Mouth

The holy grail of design is to make something so wonderful and remarkable that people can’t imagine life without it. People are so happy with it that it sells itself.
The big benefit of word-of-mouth is that your marketing budget goes toward zero, as your users become your marketers. If they’re so passionate about your design they’ll tell their friends about your service, and you won’t have to. And, most likely, what they say is more influential than what you can say anyway. Focusing on this value, and designing to enable it, is a big part of social design.
You can help enable word-of-mouth by designing your application to support it by giving your users tools to share their passion about your app or service. To actually make it work, however, you have to nail most of the following steps:

1) Get someone excited about your product and service

2) Give the sender a way to share that excitement with someone else

3) The receiver has to understand what’s being shared

4) Convince the receiver it’s valuable for them, too

5) Provide a way for the receiver to take action

6) Provide immediate value upon sign-up

7) Provide reason to return

8) Provide a reason to share with others

Liberally taken from
Bokardo