The Fundamentals Are the Same like product, pricing and promotion are same. on the other hand basic needs like mission, money, message, media and measurement are also same. Keywords Have Many Meanings. Start a Conversation. Link SEO Listings to Most Relevant Content.
Its absolutely correct that social networks play important role in site rankings as well. So we need to do it.
I would say the social bookmarking sites are extremely helpful to increase your organic search, for one they have very high PR i.e. they are authority sites
Social bookmarking sites and Social media sites both helps to increase search engine rankings and play a major role in seo.
Both facebook and twitter are famous social media sites. these help to improve ranking in SERP and increase vistior also.
Google & Bing both recently stated that they look at the quality and sources of links coming from both Facebook & Twitter. There was a good article on this posted recently at SE Land. Here is the URL:searchengineland.com/what-social-signals-do-google-bing-really-count-55389 Regardless of follow/no-follow, I think it's good to establish profiles in both Twitter & Facebook and build relationships with the existing authorities for your niches in those sites. Getting followed by the big fish in Twitter helps. I think Google is getting better at making a correlation between a website and a Twitter profile.
Yes of-course it do help you could notice that your source will have the much visibility and even received the good traffic it need. Specially if people do appreciate and follow you on your sites through social networking and other factor that could link with you.