What Social Media signals are search engines thought to be using in the SERPS?

What Social Media signals are search engines thought to be using in the SERPS?

Is it just Google+ shares/Facebook shares/Tweets/Pinterest pins etc of your WEBSITE CONTENT - or do they also take note of how many fans your Facebook/LinkedIn/Google+ company page has, how many comments/likes your COMPANY SOCIAL MEDIA PAGE has? As far as helping your main website in the SERPS goes, is having a large number of followers/fans only useful to a company in helping lots of people see (and maybe share/link to) your content or does it serve any useful SEO goal in itself? (Note, obviously, some social media pages can drive sales and leads in their own right, but my question relates purely to the SEO for your main site.)