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...
J. Griesbaum, B. Bekavac, and M. Rittberger. Handbuch Internet-Suchmaschinen: Nutzerorientierung in Wissenschaft und Praxis, page 18-52. Akademische Verlagsgesellschaft AKA, (2009)