A good way to judge that is just look at which social bookmark icons large sites provide on their sites.
Social bookmarking sites are lousey for traffic. If you're looking for a short term burst of traffic, your best bet is Stumbleupon, but your site better have something extremely entertaining, helpful or noteworthy about it.
Here it is a Top 100 Best Social Bookmarking sites, ordered by PR. Each social website have mention the type of the link they use (do-follow / no-follow / redirect) . Enjoy.
Have a really interesting content and do social bookmarking on digg, stumbleupon and more. you will surely see a great traffic