In a number of recent articles, where I've interviewed some of social media's rising stars such as Jason Stone from Millionaire Mentor, Sean Perelstein, who built StingHD into a global brand and Nathan Chan from Foundr Magazine, amongst several others, it's quite clear that multi-million-dollar businesses can be built on the backs of wildly-popular social media channels and platforms.
A search engine considers the content of the sites to determine the QUALITY of a link. When inbound links to your site come from other sites, and those sites have content related to your site, these inbound links are considered more relevant to your site. If inbound links are found on sites with unrelated content, they are considered less relevant. The higher the relevance of inbound links, the greater their quality.