It's not the ranking algorithm specifically that's a secret; it's the code that used to detect sites that work together to artificially raise page rank of anoth

It's not the ranking algorithm specifically that's a secret; it's the code that used to detect sites that work together to artificially raise page rank of another site by linking to it. The whole Page Rank algorithm is predicated on the idea that human website owners and contributors do a better job at assessing the value of a website and they express this by linking to the website. When links are created for other reasons they reduce the effectiveness of Page Rank, and when they are created specifically to bias Page Rank they do the most damage to the algorithm's results. Keeping the algorithm secret, and in particular the tweaks that eliminate links which aren't expressing "I judge this to be a good site", keep Page Rank useful.