We've covered the topic of social signals and Google way way too much. But since John Mueller of Google mentioned it in a hangout last week, I figured why not repeat what he said.
At the 19:52 mark in the video, John answered the question:
Do Social Signals have an impact on organic rankings in Google?
He said, "Not directly, no." But then he explains how if the links are not nofollowed it may influence your links, he also explains that content on social networks can and do rank in search. But overall, there is no direct impact on your own content ranking better if you get social mentions.
Here is what he said:
Not directly, no.
So it is not that there is any kind of ranking affect their. To a large part, social networks also have a nofollow on the links that they kind of provide when they post. So it is not the case that that would give you any kind of ranking boost there.
What you do sometimes see however is that the social posts do show up in the search results. They can be content like any other piece of content, and they can rank for keywords, they can rank for your product names, so they can show up in the search results as well, which in turn maybe gives you a little more presence, maybe provides some context for users as well in the search results.
Another aspect there specifically around Twitter and Google+ at the moment is that when we recognize that there is content on these social networks that are relevant to the user we will try to show that in the search results as well. I believe we show Twitter content in the US on mobile (now desktop also) so that is something that might be visible as well. It is not that your content will rank higher because of that, but there is just more content with your company name or your brand or your product name out there and we might choose to show that in the search as well.
Google has said this over and over again. There is a correlation to content ranking well in search and being shared a lot, because it is good content. But there is no direct ranking signal in Google's ranking algorithm.
Forum discussion at Google+.