Social bookmarking sites are very powerful social profiles where people can store, organize, and manage their bookmarks of web pages. High PR bookmarking sites are helping to index your sites quickly before someone steal your original content. Dofollow social bookmarking sites will earn you good quality traffic. They play a very important role in improving search engine position of your site.
Once you have created a profile on these sites, never promote your content too much, promote other content too. Because too much of a promotion will lead for spamming. Here, I suggest some most popular bookmarking sites which can increase your domain authority and improve Alexa rank by sending referral traffic.