SEO has two main elements: on-page factors, which include site content, usability, and accessibility to search engines; and off-page factors, including the inbound links that influence the degree to which search engines think a site is important.
There is no evidence that social media has any directimpact on one's search authority. Google's crawlers are unable to penetrate sites such as LinkedIn, Facebook, and Twitter, and any links that a given site gets from users on these social sites do not convey any Page Rank in and of themselves.
But social media has a large and important indirect role to play in the link-acquisition process. Here's why:
Link-Building: The Old Way
Acquiring inbound links has always been the hardest, most labor-intense tasks in off-page SEO. Because Google and the other engines prohibit publishers from soliciting links in exchange for monetary or other consideration, the only course open to a publisher is to provide original, link-worthy content, and then contact webmasters individually, usually using e-mail. While it's possible to get legitimate links this way, the process is inefficient.
Link-Building: The New Way
Social media changes the process for the better. While individual webmasters from which links are sought can still be contacted when a new article posts, the publisher's chances of acquiring links are vastly expanded. Publishers can now identify specific social media influencers and social groups relevant to the article topic, and notify them that a new page or article exists. While some targets of the notification may simply retweet, like, or favorite the notification (none of which convey any SEO value in themselves), some percentage of this audience may take the final, desired step of including a hard link to the publisher's article in their blog or site.
Social Link-Building Tips
1. Identify appropriate online groups in your industry. The people most likely to respond -- and link to an item you publish are those in your own industry. Take some time to find such groups, join them, monitor the discussion, and post a link to your article when it fits the context of the discussion.
2. Identify Social Influencers with blogs. Because your ultimate goal is a hard link that conveys SEO value, don't chase after social influencers on Twitter, Facebook, or LinkedIn who don't actively maintain blogs (not all of them do).
3. Go where the reporters go. Reporters love Twitter because its real-time nature lets them jump on news events and develop sources quickly. If you're doing something interesting (for example, releasing a new data-driven report), make sure you notify the groups where reporters are most likely to be lurking.
4. Resist the temptation to spam irrelevant articles into the conversational stream: any notifications you make must be relevant to the discussion at hand.
5. Think beyond articles. Articles you post on your blog may work as good link bait, but blog articles are pretty much a commodity today. Consider adding additional forms of content that provide more value, for example, e-books and slideshare presentations. Although you'll have to put more labor into these high-value content types, you'll be creating content that's more link-worthy.