Social bookmarking is a powerful tool in SEO because...
It may help you to build quality backlinks.Most of social bookmarking sites are now nofollow so you couldn't get link juice form all of social bookmarking sites but few of social bookmarking sites are still dofollow so you can get quality link juice and quality backlinks from dofollow social bookmarking sites.This will help you to improve your site page rank and search engine visibility.
here's few of my favorite dofollow social bookmarking sites
Technorati PR 8
Slashdot PR 8
Reddit PR 8
Dzone PR 6
Social bookmarking sits also help you to get index your new content quick as possible by search engines.because search engine crawls are always visit social bookmarking sites for index new contents and information's.