Parts of this post are premised the "influencer" marketing model, but there is at least some research indicating that it might not be as powerful as generally assumed. For example:

* http://misc.si.umich.edu/media/papers/wsdm333w-bakshy.pdf

* http://www.digitaltonto.com/wp-content/uploads/WattsandDoddi...

"Grouped: How small groups of friends are the key to influence on the social web," the book by Paul Adams @ Facebook (http://www.amazon.com/Grouped-groups-friends-influence-socia...), talks a bit about this.

Anecdotally, I've also seen quite a bit of evidence first hand that suggests the impact of influencers is not clear-cut.