The significance of social signals as a ranking factor is an ongoing topic of discussion in SEO. Google never gives full details of all known ranking factors but does state that 300+ signals influence SEO and keyword ranking. We always talk about SEO strategy across a broad set of categories (Content, technical, backlinks, on-page, off-page, etc) which likely covers the 300+ signals.
But when it comes to a competitive industry, we need to consider that the key ranking factors are those that provide the best user experience, of which social media is a strong signal. It is straightforward to win for branded terms in search results by setting up social media accounts with vanity URL, title, description, share, tweet, pin, etc.
From officially published research, social signals influence SEO and ranking by 3-4 %, but there are many more direct and indirect co-relations between SEO & Social.
For example  on Facebook, a fan page can be optimised by:
  • Adding an SEO title = Your fan page name
  • Meta description = fan page name + the about section of your page
  • H1 tag = your fan page name

Amends such as these can help to improve site ranking in Google search results pages.

For another example, Google has partnered with Twitter to index the tweet, retweet, etc., which helps them to identify the authority, reputation, relevancy, etc. of posts.
 We believe social media influences SEO directly and indirectly, in the following ways:
  • Brand term ranking with social media
  • Securing additional backlinks
  • Driving traffic
  • Building the site authority
  • Improving dwell-time, decreasing bounce rate, etc. by sharing engaging content via social media