Social bookmarking is a very popular form of backlink building also to get some direct traffic from the social bookmarking sites too. These sites are used by a massive number of people everyday and Google crawl these sites almost everyday. Sometimes multiple times in a day. If you bookmark your sites into these bookmarking sites, there are high chances google will index your website's pages quickly also will count those bookmarks as a backlink of your site which will help you to get a better search engine ranking.