Very few. However, although not clicking on a link to immediately buy something, clicking a link introduces you to a site you might, otherwise, not have known about. I never used to read the New York Times or the Huffington Post, but the depth and perception of the articles they have linked to on Facebook has made me a semi-regular visitor of both sites.
Moreover, using the viral marketing of Social Media helped the Huff Post become an attractive asset for AOL. Instead of buying it, breaking it, and closing it, AOL saw its value and continues to build it. Building a web presence on Social Media also helped writer Andy Borowitz get a gig at the New Yorker, a magazine that paid attention to him because the rest of the world was attentive to his writing.
It also has enormously helped the sales of writer Tom Ryan's book "Following Atticus". He probably doesn't get a whole lot of direct sales from clicks, but those clicks introduce him to a national and International community better than any author PR agency could without spending a huge amount of money.