Since I'm a student of referrals and the word of mouth process, I was delighted to see a research-based attempt to explain the process in the new book by Jonah Berger entitled Contagious, Why Thing Catch On. He asserts that there are six components to things that go viral. The first is "Social Currency," sharing things that makes us look good. "Triggers" describes the importance of top of mind. "Emotion" is the share-because-we-care component. "Public" suggests that public consumption has more contagious possibilities than something used privately. The fifth is "Practical Value" and the sixth is the use of "Stories." Replete with lots of examples!