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...
"Well, I've been buying Google search terms for about two years now, but the price keeps going up. I have a feeling that my competitors are clicking on my link just to drive my costs up." Gary parked the van to let me out, and dove straight for his Blac