Network-based marketing

Network-based marketing refers to a collection of marketing techniques that take advantage of links between consumers to increase sales.

A study, co-authored by Shawndra Hill, Wharton professor of operations and information management, found that consumers are far more apt to buy a company's product if they are "network neighbors" with existing customers. Mining data from "social networks" -- who talks to whom or who emails whom -- could allow companies to pinpoint likely customers who otherwise would be overlooked.

The full paper, "Network-Based Marketing: Identifying Likely Adopters via Consumer Networks" can be found at http://arxiv.org/PS_cache/math/pdf/0606/0606278.pdf

This explains why social networking sites like myspace, youtube...etc attract huge valuations.
eBay recently purchased Internet-telephony upstart Skype for $2.6 billion; they now also will have large-scale, explicit data on who talks to whom.