I too agree with RH-Calvin, that social bookmarking sites are useful only to get traffic to the website and from my point of view it doesn't help out in ranking any of the keyword of the website.
Social bookmarking only provide traffics, but some high rank social bookmarking sites provide good back links not all SB sites.. Not all Social bookmarking sites good for link building, many SB sites are low quality & spamy..
Can Social Bookmarking boost your search engine rankings?
Social bookmarks are a way of storing, managing, and sharing publically links to your favorite content on the Internet. That means that others searching for the same kind of information can see which resources you found useful and have bookmarked. Tagging and the use of hash tags, takes bookmarking one step further and allows users to organize their bookmarks and make them searchable. There are many social bookmarking sites with many features, including Pinterest, StumbleUpon, Delicious, Digg, Reddit, Newsvine, and many more.
There are many social bookmarking sites with many features, including Pinterest, StumbleUpon, Delicious, diiigoo.com, bookmarkscity.com, city4webs.com ...etc
It is strange, how can you get traffic from social bookmarking? If you can do that, well it really helps you rank your keyword. I think the most importance is traffic.
Social bookmarking is an important off page seo technique to get website traffic and website ranking.But make sure always use high pr sites to submit your website content.