The Official Answer: Correlation ≠ Causation
Here’s what Matt Cutts (head of Google’s webspam team) had to say:
“Just trying to decide the politest way to debunk the idea that more Google +1s lead to higher Google web rankings. Let’s start with correlation ≠ causation… If you make compelling content, people will link to it, like it, share it on Facebook, +1 it, etc. But that doesn’t mean that Google is using those signals in our ranking.”
At SMX 2014, Amit Singhal (head of Google’s core ranking team) again refuted this idea:
Amit Singhal: That’s right, we’re not using [social signals from Twitter of Facebook] right now. We don’t have access to the Twitter data, so you can imagine how hard it would be to build a system that relies on those signals.
Danny Sullivan: You do have access to Google+, but you’re not using that for impacting the unpersonalized results?
Amit Singhal: Right. We think about it from a user’s perspective. They’re looking for high quality content generated by reputed people. We have the authorship program that allows that to happen. They’re also looking for content from people they know, and we have the personalized results for that.
Still, There’s Clearly Correlation. So Why Is That?
01. Social signals correlate with publishers’ popularity
…which in turn correlates with the signals that search engines actually value. An article on the New York Times is much more likely to get shared than your average blog post by virtue of having built up a huge readership over the past 163 years, if nothing else. It’s also much more likely to rank well — not because of those social shares, but because of the incredible backlink profile and authority metrics that go along with building an institution like the New York Times. Social shares and strong rankings are more symptoms of success than its cause, in that context.
02. Social media can be a link building channel
As we all know, search engines love links. Social media can be a powerful way of promoting content. The key is promoting your content to other content creators, who might then reference it in their next blog post. That makes social one of the best link building channels (especially now that Penguin is in effect), but it also puts the focus on tapping influencers rather than audiences if organic traffic is your ultimate goal. So the question you should ask yourself isn’t “how many shares can I get?” but rather “where are the influencers in my industry and how can I get them to link to my content?”
The Bottom Line: Use Social to Reach Influencers
Social can still be a useful tool for SEO, but more as an outreach tool than a way to game the algorithm. Even using it as a community building exercise isn’t a great approach if you’re focused on organic rankings and traffic.
Of course, SEO is rarely a silver bullet and that kind of tunnelvision does not a thriving company make. Social media can be used to reach both influencers and potential customers; using it purely as an SEO tool neglects its value in doing the latter and a balanced strategy is best for most organizations. 
