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My understanding is the C-block bar has nothing to do with social factors, but instead is being used as a control measurement. Prior to social sites becoming so popular and other algorithm changes, the strongest off-site correlation for Google ranking factors was the number of linking C-blocks to a website. The graph is being used to demonstrate how Facebook shares and other social metrics are so incredibly strong they overshadow the number of linking C blocks metric. That is how I interpreted the graph. If anyone else read it differently I would welcome the feedback.