Social media impacts SEO in two ways. Firstly, Google has a section dedicated to shares of content on line and secondly, all search engines have admitted that social media has an impact on search results in one way or another. Based on some anecdotal evidence, the following is confirmed about social media and SEO.

• Tweets from real users impacts on search
• There is probably a connection between the authority of a user, and the value of the tweet
• There could be a long term value to tweets

Google says that they do use Tweets as a signal. It is used as a signal in our organic and news rankings. We also use it to enhance our news universal by marking how many people shared an article. Both Google and Bing admit they do the same for Facebook, although they have no way of getting personal data. So if information is shared widely, it will rank better. They also give indications that if an article is shared in both places, it is likely to be legitimate, and therefore will rank better.