Market structure

In 2006, North American advertisers spent US$9.4 billion on search engine marketing, a 62% increase over the prior year and a 750% increase over the 2002 year. The largest SEM vendors are Google AdWords, Yahoo! Search Marketing and Microsoft adCenter. As of 2006, SEM was growing much faster than traditional advertising and even other channels of online marketing. Because of the complex technology, a secondary "search marketing agency" market has evolved. Many marketers have difficulty understanding search engine marketing and they rely on third party agencies to manage their search marketing. Some of these agencies have developed technology that automates bidding and other complex functions required for the Pay Per Click model.

Page Rank

For search-engine optimization purposes, some companies offer to sell high PageRank links to webmasters. As links from higher-PR pages are believed to be more valuable, they tend to be more expensive. It can be an effective and viable marketing strategy to buy link advertisements on content pages of quality and relevant sites to drive traffic and increase a webmaster's link popularity. However, Google has publicly warned webmasters that if they are or were discovered to be selling links for the purpose of conferring PageRank and reputation, their links will be devalued (ignored in the calculation of other pages' PageRanks). The practice of buying and selling links is intensely debated across the Webmaster community. Google advises webmasters to use the nofollow HTML attribute value on sponsored links. According to Matt Cutts, Google is concerned about webmasters who try to game the system, and thereby reduce the quality and relevancy of Google search results.