Links
Links are the glue that tie the internet together like synapses between neurons in your brain, and like a brain, these links are constantly made, broken or remade. 
When search engines look at links counts, they are essentially looking for editorial votes for the importance or popularity of a site. Unlike a traditional voting system however, it is not one vote per person. The more important the site, the higher weight associated with the casted vote.
In the past, link building involved getting other sites in your vertical demographic to link to you, however with the explosion of social media, there are whole communities available for you to connect with. Build relationships with them, give them a reason to talk about you and you can harness a power far beyond the meagre link swapping approach.
Shifting approach
Of course as you might expect, successfully tapping into social media requires a very different approach. In fact each aspect of social media requires a different skill set. For example, working with blogs is an entirely different strategy to creating a presence on sites like Digg.
In order to harness these aspects of social media, it is not simply good enough to turn up to a site and drop links. You must be able to interact, which means you must be able to understand how to engage, provoke and make friends.
Before you attempt to engage with social media, make sure you understand how it works. Make your own forums posts and topics, create your own blog and engage with other people. The power of your links will come from the authoritative people who link to or follow you.