NB: Tweeting used to help in getting content indexed. Re-tweets helped it to get indexed quicker. Alas, since Google stopped paying for the full fire hose of Twitters data this no longer appears so (especially if you only get small numbers of RT's). It's still worth doing, but don't hold your breath.
If you create shareable content it'll get passed around, and (more importantly) linked to. Putting the time in to creating video, infographics, audio, a bit of creative meme riding, finding and crafting the right images - content like this is fried gold if you want people to organically link to your posts or site. There's no substitute for the well researched and nicely presented content that pushes your audiences buttons. Embedding that into blog post and into other services promotes sharing and linking.
Link baitLinking to Channels
For SEO, forget fancy embed codes from the likes of Facebook on your main site (yes, this goes against all the best community building practice, I know. Saying this feels like playing Tie-Fighter after a month of playing X-Wing, just dirty). The Facebook embed code, even if it does promote sign-up by showing your friends who already like a Page, slows a site down so much it's like watching stoned turtles stampede through peanut butter. Go with direct links, above the fold, and a clear 'trawlable' line of access to the social channels. Also, tag them. Putting alt text in place that says the likes of "Follow [blah] on Twitter" is another way to get the brand name in there, and is great accessibility practice.
Get on G+, and Develop Your AuthorRank
Google Plus is getting better. No, really. Not only is it getting more and more usable but Google is making it valuable for search rankings. Posting your blog content here, and actively going about redistributing it by promoting +1's and developing a community is 'where it's at.' How you do that is a whole book in it'self, but starting now with basic set-up and posting is a positive way forwards. Google Plus also seems to promote rapid indexing and trawling, with any 'shares' and '1+s' meaning it's trawled and indexed even quicker.
In the next few years AuthorRank has the potential to become the backbone of credibility. Get your G+ profile linked to your blog content. Now. Start building your reputation. Now. For more on this check my 'Why You Can't Ignore AuthorRank' post from back in January. AuthorRank is like PageRank for individuals, and it's all about 'social signals'. Once you've got your profile linked to your blog you can start to generate credit for the content you create - the 'scoring criteria' runs on factors like 'likes', 'shares', 'retweets', 'comments', '+1s' and the influence of the folks (your followers) doing the sharing. Get this sorted now, you'll thank me for it in 2014. Rel=Author tags are important, and the more you write on a given subject and the more 'love' it gets, the more Auntie Google will pimp your stuff.
In general, Pinterest seems to be good for inbound links. General good practices on Pinterest (adding prices, linking across platforms, filling out the whole profile, fresh content, linking to individual products, categorising boards, adding keywords to descriptions, etc.) seem to be a real boon, and we've got clients that pick up PageRank from being connected to their higher PageRank Pinterest accounts. Every little helps, right?
Write Some Original Quality Content
Not average run-of-the-mill content. Good original content, just like the link bait above. 2-3 fresh posts a week and Google will love you for it - though bigger brands will need to do this a lot more often (probably as much as 2-3 times a daily). Create posts people want to share and link to. Share what you know - remember, you're not giving away information your spreading your knowledge and enhancing your credibility. One blog post that gets shared is worth the time it takes to write 6 blog posts that just sit there gathering social dust. People like to share good useful information from sources they trust that's helped them or they believe will help other people. Often this will be about those common interests and the answer questions they didn't even know they needed an answer to - go take a look at Quora or Yahoo Answers and have a search to see what folks want to know. If it's new, funny, topical, or bordering on the obscure, people love it. Think about working in those compelling (and actionable) calls-to-action and tantalising headlines, and don't cram articles with keywords (those days are gone). Keep your content relevant and accurate, and grow your credibility as an authority. Stimulate debate. Be awesome. Rock your peers. Get writing, but make sure it's decent stuff.
If sharing is good for content distribution and you want to up your 'social signals', make sure it's easy for the reader to share (yeah, I know, the theory's hardly rocket science). There's a few good services for this that add all the relevant (the key word here is 'relevant', no one wants to wade through a bazillion icons) social shearing features - try AddThis, ShareThis, or Lockerz - they all do the same thing in different ways.
Also, just as a point, if you spend a few pennies to promote your social posts and get them under the nose of folks who will share it, well that's just good 'white hat' tactics ;)
sharing is good
All this is just scratching the surface. If you want to boost your SEO through distributing content via social media this is a good start. How you get people to do that is another story, but putting the back-bone in place so that they can is an excellent start. Quality and access, that's all it takes.