By 2004, search engines had incorporated a wide range of undisclosed factors in their ranking algorithms to reduce the impact of link manipulation. Google says it ranks sites using more than 200 different signals. The leading search engines, Google and Yahoo, do not disclose the algorithms they use to rank pages.
Notable SEOs, such as Rand Fishkin, Barry Schwartz, Aaron Wall and Jill Whalen, have studied different approaches to search engine optimization, and have published their opinions in online forums and blogs. SEO practitioners may also study patents held by various search engines to gain insight into the algorithms.
In 2005 Google began personalizing search results for each user. Depending on their history of previous searches, Google crafted results for logged in users. In 2008, Bruce Clay said that “ranking is dead” because of personalized search. It would become meaningless to discuss how a website ranked, because its rank would potentially be different for each user and each search.
In 2007 Google announced a campaign against paid links that transfer PageRank. On June 15, 2009, Google disclosed that they had taken measures to mitigate the effects of PageRank sculpting by use of the nofollow attribute on links.
Matt Cutts, a well-known software engineer at Google, announced that Google Bot would no longer treat nofollowed links in the same way, in order to prevent SEOs from using nofollow for PageRank sculpting. As a result of this change the usage of nofollow leads to evaporation of pagerank.
In December 2009 Google announced it would be using the web search history of all its users in order to populate search results .
Real-time-search was introduced in late 2009 in an attempt to make search results more timely and relevant. Historically site administrators have spent months or even years optimizing a website to increase search rankings.
With the growth in popularity of social media sites and blogs the leading engines made changes to their algorithms to allow fresh content to rank quickly within the search results. This new approach to search places importance on current, fresh and unique content.
2011 came the rise of Penguin and Panda. Google Penguin filters and penalizes websites with low quality backlink profiles or those who acquire links from shady sources. On the other side, Panda is intended to filter out low quality content.
2015 came Mobilegeddon where Google favors websites with good mobile compatibility.
In late 2015, Google announce the rise of hummingbird and Artificial Intelligence. This was a complete overhaul to the whole algorithm system where Google analyzes the search intent behind every query.
This changes the whole SEO game as the days of giving results that was based in word per word system is completely over.