Search algorithms & popularity factors: link, social, click, blog, industry. Since modern search engines are concerned with popularity and not direct relevancy, & big firms up the price of text link buying beyond affordability...
R. Jones, R. Kumar, B. Pang, and A. Tomkins. CIKM '07: Proceedings of the sixteenth ACM conference on Conference on information and knowledge management, page 909--914. New York, NY, USA, ACM, (2007)
M. Hildebrand, J. van Ossenbruggen, and L. Hardman. International Semantic Web Conference, volume 4273 of Lecture Notes in Computer Science, page 272-285. Springer, (2006)