In The Search, John Batelle discusses how Google's success came from Pagerank, which measures the engagement of documents to documents.

Wouldn't it be interesting if, rather than measuring engagement of documents to documents, you did so for people and topics? It appears to me that what is finite in digital media is attention, not content nor available ad units. The real trick for marketers, however, is to map a person's preferences to the traffic they are viewing--not another document's preferences.

Today there is a ton of uneditorialized traffic that is engaging millions of people, but isn't engaging other documents the way earlier incarnations of the Web did. While the content on the page may be great, the context is very narrow--usually just the interests of an individual. The trick is to find a new way to aggregate this traffic.

Behavioral targeting is the answer. You do not need the content of millions of personal pages and blogs to be referenced by other documents because behavioral targeting enables marketers to engage people based on their personal context rather than that of pages. In essence, behavioral targeting is a superset of intentions beyond search that extends the usefulness of search advertising and enhances it by incorporating other sources of intention. Search knows pages or words and their engagement with other words while behavioral targeting knows people and their engagement with topics. Understanding this complementary relationship is critical to getting maximum value from both strategies.