Out of sheer curiosity, the great team at Compete used their demographic data to craft a mini-infographic showing the income breakdown across 6 of the top social sites. The US Browser Population is lined beneath each data-set for further comparison and context.
Of these properties, LinkedIn shows the most notable emphasis towards a specific demographic, individuals earning 60k and above. Pinterest shows a slightly higher representation among higher income demographics and Tumblr shows the same slight skew towards less affluent demographics.
Facebook and Google+ show even representation between all demographics, probably a result of how pervasive they are across the social-scape.