Why do networks work?
One of the original roles of advertising was simply to convey information - availability, features, pricing, etc. In an iMedia article Cindy Gallop makes the case that marketers no longer are the fine purveyors of information. They have been replaced by a variety of nearly instantaneous sources.
People in the search or consideration stage turn to numerous resources for information, clarity, and reinforcement of their own opinions. If the resources are highly interconnected they are likely to a) share the same information often reinforcing the message and b) spread the information quite rapidly. These cliques (yep, that's the technical term) can either support a point of view or provide contrarian views.
For this reason understanding the flow of information and the architecture of influence is a new discipline for marketers.