One of the most popular forms of web spam, manipulative link acquisition relies on the search engines' use of link popularity in their ranking algorithms to attempt to artificially inflate these metrics and improve visibility. This is one of the most difficult forms of spamming for the search engines to overcome because it can come in so many forms. A few of the many ways manipulative links can appear include:
- Reciprocal link exchange programs, wherein sites create link pages that point back and forth to one another in an attempt to inflate link popularity. The engines are very good at spotting and devaluing these as they fit a very particular pattern.
- Link schemes, including "link farms" and "link networks" where fake or low value websites are built or maintained purely as link sources to artificially inflate popularity. The engines combat these through numerous methods of detecting connections between site registrations, link overlap or other common factors.
- Paid links, where those seeking to earn higher rankings buy links from sites and pages willing to place a link in exchange for funds. These sometimes evolve into larger networks of link buyers and sellers, and although the engines work hard to stop them (and Google in particular has taken dramatic actions), they persist in providing value to many buyers & sellers (see this post on paid links for more on that perspective).
- Low quality directory links are a frequent source of manipulation for many in the SEO field. A large number of pay-for-placement web directories exist to serve this market and pass themselves off as legitimate with varying degrees of success. Google often takes action against these sites by removing the PageRank score from the toolbar (or reducing it dramatically), but won't do this in all cases.
There are many more manipulative link building tactics that the search engines have identified and, in most cases, found algorithmic methods for reducing their impact. As new spam systems emerge, engineers will continue to fight them with targeted algorithms, human reviews and the collection of spam reports from webmasters & SEOs.
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