Search engine optimization is a constantly-evolving mix of science and intuition. The search engine algorithms are only loosely-defined by their creators and constantly changing. What we can say definitively is that the old days of keyword stuffing and backlinks are long gone.
These days there is more of an emphasis on quality content. How long people stay on your site, number of page views, bounce rate, how many people link to your content from their site, how many people have shared the content on the major social sites like Twitter, Facebook and Google+? These aspects are given more emphasis.
Social media activity and well-executed paid search activity is what moves the needle on page rank.