The term social bookmarking can be misleading for someone who is not particularly familiar with web content development and marketing. It's got nothing to do with the regular bookmarking of websites or web pages that we do by clicking the star icon at the extreme right-hand corner of the URL bar.
Actually, social bookmarking is the term used for content aggregation sites where a cross-section of bloggers, SEO professionals, and journalists among others post their content for better distribution and ranking on search engines. Some of the popular social bookmarking websites are Redit.com, Tumblr.com, and Digg.com among others.
Now that's easier said than done because many of these sites including those that I mentioned here, won't accept content from just about anyone. They only accept content from 'high authority' contributors which mean, you have to win their trust and respect as a regular member of their sites where you do not just contribute your own content for better distribution and ranking on search engines but also read and comment on content posted on the site by other users. In other words, it's not one-way traffic; you will have to show some commitment toward the site and enrich it by engaging with its interactive features.
In addition to posting your content for better distribution and ranking on search engines, social bookmarking sites also allow you to increase backlinks to your website. You need to remember that hyperlinking keywords beyond the permissible density could render your content as spam and most top-ranked social bookmarking sites are quick to pick up spam content on their radars. The other thing that you need to watch out for is that on social bookmarking sites, your content will be read and evaluated by other users and many of them are SEO experts having a cobra's nostrils can smell spam from a long distance. Hence, utilizing the advantages of social bookmarking sites wisely for the best results.