I have been studying various viral marketing, word of mouth marketing, idea diffusion, creating tipping point papers, lately. People are trying to build an explanation that I presume started with TIPPING POINT by Malcolm Gladwell in his mid 1990's book of the same name.
Recently researchers are talking about a measurement metric called Net Promoter Score. Although it is not standing the test of time as can be seen in the recent papers - especially in the Journal of Marketing paper.
It may be of interest for readers to explore some of the papers linked below
Net Promoter Score Under Fire
Value of different Customer Satisfaction studies
A very interesting collection of Net promoter score papers and discussions
More than 1000 pages tagged Word of Mouth Marketing
Is there a strategy for creating EFFECTIVE WOMM - just wondering?