It can be Bold Seo/ White hat Seo, Which return you organic results. Social Bookmarking site are the major cause of ranking fluctuations. http://www.knowledgeidea.com/social-bookmarking-sites-list/ Remember that, Ranking strategy would be impact on your Organic Traffic/Search.
Generally Social networking sites are good source of generating traffic it depends upon on the target audience or the geo traffic in India Facebook will work apart from the other sources for professional stimulation linkedin will work like wise...
India, Pakistan, Bangladesh, Sri-Lanka like countries are emphasize on social traffic because Social media sites have tons of traffic in capacity. Likelihood in few years, Huge segment of internet users will be available on social media.
depends on what kind of traffic you want to get. Facebook has more unversal audience so generally its traffic is better
Depends on the niche of your website and traffic you get. If your content have a lot of images Pinterest should work for you. Fb, G+ and Twitter are universal as phyodor says.