Why links matter
Incoming links from authoritative sites have long been an important part of SEO. They are akin to "votes" that signal a page's relative popularity and relevance to a topic. But search engines don't merely count links, they look at the topical relevance of the pages that link, the authority of the sites that link (most likely gauged by how many links point at these domains), and the anchor text of the link ("ecommerce blog" is more valuable than "Get Elastic" or "click here" in links pointing to this blog.)
However, links that are appended with a "nofollow" attribute are not followed or counted by search engines. Webmasters may add a nofollow attribute to an ad, user comment, or link to a site of questionable quality. Because social links are easy to spam, they typically are nofollowed. But it turns out, that's not a problem. Both Bing and Google have access to a "fire hose" of data from Twitter, including links without the nofollow attribute.
Why authority of social sharers matter
Just like search engines assign higher value to links that come from more authoritative sites, they assess the authority of Tweeters and Facebook pages that share and re-share links. Bing reportedly factors the number of times a story is retweeted along with the authority of the tweeters (number of followers/following, etc.) Google also factors a tweeter's authority, and eludes to using this data for its News search and "limited situations" in web search. Both engines factor Facebook links, but only for pages that are public like Fan Pages or public Walls.
So what does this mean for ecommerce?
1. Make it easy to share
Those little Twitter and Facebook Tweet, Share and Like buttons you see on more and more commercial sites make it easy for social networkers to share your content. You can add them to both your home page or to product pages.
You can also include calls to action on your home page to Fan or Follow your Facebook or Twitter pages, respectively.
And don't forget emails...
It's tough for ecommerce sites to build natural (unpaid) links, so presenting these calls to action to visitors can win you "passive word of mouth" and "passive link building" - you never know who is a social "whale" that might retweet or share.
(You can find a directory of Facebook plugins here).
2. Reach out to influencers
Extend your PR efforts to Twitter-ers, just like you would to bloggers and other media. Yes, that involves some research and perfecting your pitch to Twitter whales. I wouldn't bank on Sponsored Tweets, as it's easy for search engines to detect the word "sponsored" or "ad," and most Twitter-ers will disclose paid tweets as such. You can still sponsor tweets for advertising, but it won't help your SEO.
3. Become an influencer
Build up a strong fan base and Tweet / update Facebook regularly. Leverage your site and emails to ask for the follow / friend.
Be sure to tweet and add to your Facebook page things your followers would be interested in (topical stories related to your industry, fun facts, videos, funny images, etc), not just links back to your own site.
Following back your followers may help boost your authority ranking (at least, that's what I gather from Bing's response), so it doesn't hurt to reciprocate. There are automated, paid tools like SocialOomph that can help you with this if you can't keep up manually.
Incoming links can help boost the Page Rank of your social profiles, so include links to them on your site and ask other sites that may be linking to your home page to include a link to your social networks. For example, if I'm interviewed by another blog I will ask if they include @getelastic and @roxyyo in my byline.
You may wish to invest in a full or part time social media manager who can keep the communication flowing through your social profiles.
4. Shoot for viral
Think about what content on your site has potential to "go viral" on social networks. It could be a contest, a funny/groundbreaking video or breaking news story. It's not easy, but ecommerce sites like Office Max' Elf Yourself app or Best Buy's current Twitter contest are examples.
Are social links a magic bullet for SEO?
Not quite. Social links make up only a small part of what search engines factor when ranking pages. But I suspect they will only become more important, as search engines begin to incorporate more social content into search and refine their methods of finding authoritative Tweeters. It's a good idea to take advantage today, and reap the word of mouth and search engine benefits Twitter and Facebook can offer.