It is difficult to come up with hard & fast rules to measure effectively a still evolving technology/trend/community's behaviour & dynamics..
but "Meeyoung Cha" 's work is a good start and reported above on HBR.
Key however still is how 'trusted relationships is measured and how these people are listened to and their advices/suggestions are acted upon.
e.g. if Oprah says buy a book, millions of american would buy it, but when richard & judy says buy the book, maybe only hundreds/thousands?
So, it seems to me, its still as difficult as measuring the 'fellowship' on the traditional media world.. but surely on the digital world, we should have a better way of doing it..
It maybe we need to find a new KPI that is applicable (no more eye balls please!).. and no, not followers either!