According to Google, Social Media likes and shares are not taken into account when Google presents web pages in a search engine result page (SERP).
In Aug of 2015 John Mueller (also of Google) said the same thing.
In short, social media does not DIRECTLY contribute to your sites SEO.
Are there any ways social media INDIRECTLY contributes?
Consider that both links to pages on your website and comments on your blog posts have a positive benefit on your SEO (I think the SEO benefit of links is well known – the benefit of blog post comments is sites that are updated more often tend to rank higher and every blog post comment is an update).
Social media shares expose your posts to more people. Some of those people like your stuff. Some of those people share it via social media thereby exposing it to even more people.
But the real SEO benefit is some of those people like your stuff enough to link to it and comment upon it, and those activities DO have a direct positive effect on your SEO.SEO Needs Link Building