"People are more likely to trust a recommendation from a friend or a family member than by a search engine. This means that if the search engines aren’t embracing social media, they are going to lose out big time.
With this, Google integrated social media into their core search algorithm and in 2010, the famous Matt Cutts announced that Google is using “social signals” in their search algorithm, and they have been doing this for a while. Social signals play an important part in increasing website traffic and to obtain organic rankings and have been doing so for a while, but many are only starting to realize this now.
What does this mean? It means that SEO has evolved once again and is now working hand in hand with social media".
Via Antonino Militello, Jojanneke v.d. Bosch