Firstly, it is worth bearing in mind that if a searcher is getting personalised results (if they are signed in) and they are a fan of your business, it is highly likely that you will appear higher up their search page. The same goes if they have been visiting you a lot. So it is worth getting them to your site often as that will increase the chance that they will come back even more!
But, I think that we need to look at this from a non-personalised point of view.
It appears that one of the things that Google found in a recent study on social sharing is that it is inherently more PERSONAL than broadcast channels and as so may not be as big of a ranking factor as some people think.
On the other hand, a huge study performed by Search Metrics: http://blog.searchmetrics.com/
showed a strong correlation between social signals and Google rankings. (correlation, not necessarily causation)
It is likely, that social metrics are used to factor into a trust and authority score in some way but not all of them. Google have recently come out and said that +1 are not a good quality indicator at the moment. If you think about it, Social media and the platforms it uses change so quickly that if would be very hard to use indicators such as likes or Tweets to influence rankings because of how fast the landscape changes.
To the extent that Google can identify users, it may have an effect on rankings, but probably not the individual sites.
Social media is of huge benefit to a business but probably not from an SEO point of view.
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