Using links as an Off-The-Page ranking factor was a great leap forward for search engines. But over time, links have lost some of their value for a variety of reasons. Some sites are stingy about linking out. Others block links to help fight spam. And links get bought and sold, making them less trustworthy.
Enter social media. If links were a way for people to “vote” in favor of sites, social media sharingrepresents a way for that voting behavior to continue. Social signals are emerging as ranking factors as search engines determine how to leverage our social interaction and behavior.