Both Social Bookmarking and directory submission are different Off-page SEO techniques.
Directory submission- It is used to get high rank of our website through quality back links. Through it, you can boost the ranking of our website in the search engine. It is effective as a tool to increase the traffic volume on our website. But back links should be create on high PR site Social bookmarking is more beneficial to get traffic on website. But we need to focus on major issue in bookmarking of C-class IP, so never try to get backlink from C-class websites