Links from sites like facebook and twitter are important as Google treats them just like any other Read full post from How Does Google Rate Links From Facebook & Twitter?
Social Media Sites are commonly accessed by lots of users. Therefore, it could somehow contributes for the ranking of your site because if you're sites are linked to these top visited social media sites there's a chance that your site could be reached or open by the users. For me, even if small things should be used as long as it could bring good impact to on my internet marketing strategies.
Well Social media marketing definitely increase traffic to your website and it is a powerful strategy that will get you links, attention and massive amounts of traffic.
Social Media increases traffic to your site, but when speaking of link building it would be better if you try to get links from various sites and various types of sites
Twitter and facebook links are no-follow. But google count that too, and giving some weight for that links.
Links from social bookmark can be both "Yes" and "No" in its important. Most of these links are easily to manipulate even with a hard moderation by their side. It has some important and can be "partial" in link building efforts but not a key roll anyway.