Yes definitely social bookmarking improve SEO. Use high PR social bookmarking sites, it will generate traffic and improve your keyword rank on SERP.
Facebook and Twitter do not provide any link juice. But they are very potential for getting huge traffic. If your site is useful and people like it, then they will help you increasing the popularity of the site. Which is good for SEO.
On first hand you are 100% right. But users can read about your tweets, go to the original content if any, and might like it and link back to your site, so in this case it is good for SEO to tweet and other social things.