SearchMetrics conducted a study recently on Google's search engine rankings and how Penguin 2.0 affected those. One of the key takeaways was that social sharing continue to correlate very well with better rankings.
I look at this way: Everytime someone tweets or shares my content it acts as a "vote" in the "popularity contest" that is Google. So everytime someone shares what I've created they are essentially saying they approve and whatever I wrote is worth reading. Google takes that into account when it is serving up SERPs.
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