Now a days Social networking sites are very much help for SEO, because Google like a all top search engines are crawled those social networking sites regularly.So once we have uploaded with quality content then your websites also get popular.
social network is not directly help SEO,depends which network you use. If you use good social network like FB or Google+,your site will automatically ranked. But SN is a powerful source of IM and more traffic. SEO is totally another term that help you to rank your page,but it needs search engine. Without search engine it is dead.
yes you can get traffic by social bookmarking. Social bookmarking is a method for SEO to search, to manage bookmarks of web pages. you can build web and promote site
I think, as long as you build informative content on your social networks, it will attrack a lot of visitors and boost your targeted website!