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Rand Fishkin recently blogged on the effects of Google + and ranking performance. Video included... recommended.
Google knows how much of your traffic is coming from social sources.
It used to worth doing all this for Facebook traffic. This days when a user likes your site only a small fraction of the user's friends see the like in their feed. I know this because the number of likes my site pages get has been increasing over they years while the referral traffic I get from Facebook seems to be caped to a certain amount.
If I correctly understand what Rand Fishkin is saying, then apparently anyone with a lot of G+ followers could post a piece of worthless garbage, and it would soon start showing up at the top of those followers' search results.