Paid direct links are a challenge to way the major search engines do business. Engines' ranking algorithms are based on the concept that a link from one site to another is a vote that conveys reputation and general relative relevance. Google's PageRank, for example, is driven to a great extent by linking relationships.

The other major search engines use link analysis to assist in determining relevance and rank, in addition to page content analysis. The truth is page content often ties with relevance. Link structures -- including the anchor (keyword being used to link) -- the sites doing the linking, and the relative number of links on a page all factor heavily as tie breakers.

No wonder, then, in the last several years marketers have found themselves tempted to buy direct links that can deliver relevant traffic and SEO benefit simultaneously. Text-link networks are proliferating, and some SEO firms have under-the-radar link networks they don't advertise publicly because the engines are starting to invalidate links they believe are paid. Ironically, these link networks are often channel-based by category, making them similar to the contextual ad networks search engines endorse.

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