I'm sure that I'm oversimplifying this point, and anyone who would like to straighten me out is welcome to comment, but to build search traffic to your web site, avoid posting directly to social media sites. It does take some practice to reliably syndicate your blog posts to Facebook and Twitter, especially if you want to make the post enticing to read. If you are interested in building reputation and traffic for your web site, the content should originate there and permanently reside there.
SmartBlog On Social Media: Do Social Media Links Help Your SEO? 2010-June-2, by Rob Birgfeld
Many of the biggest social-media sites apply what’s called the “nofollow” attribute to their outgoing links. This tells the search engine robots that periodically crawl a page and report back to not follow a given link. In other words, the SEO benefits mentioned above – commonly called “link juice” – don’t transfer over. Google strictly adheres to this; Yahoo! and Bing, less so. Users can click the link and visit the target page, but the link itself will not help the site in search results.
Facebook and Twitter use the nofollow attribute, as do YouTube, Digg and Flickr. ... Most of the big social-media sites don’t pass along any direct search benefit from their high ranking.
... [Except in their RSS feed format] ...
And all the major blog-hosting sites – WordPress (.com), Blogger, Tumblr, etc. — do not nofollow, or “dofollow,” if you prefer. Links from hosted blogs and self-hosted blogs, if relevant, will help rankings.