The benefit of using social bookmarking site is:
Indexing Sites Faster : Humans bookmark sites launched by their friends or colleagues before a search engine bot can find them.
Deeper Indexing : Many pages bookmarked are deep into sites and sometimes not as easily linked to by others, found via bad or nonexistent site navigation or linked to from external pages.
Measuring Quality : Essentially if more users bookmark a page, the more quality and relevance that site has. A site with multiple bookmarks across multiple bookmarking services by multiple users is much more of an authority than a site with only several bookmarks by the same user.
External Meta Data : Users who bookmark sites tag them with keywords and descriptions which add an honest and unbiased definition which is created by the public and not the owner of the site.
Co Citation : Social bookmarking sites tend to categorize sites and pages based upon the tags used by humans to describe the site; therefore search algorithms can classify these sites with their peers.
Number of Votes : Similar to the number of bookmarks, the more votes a page receives on Digg or Reddit, the more useful that information usually is. If the same page receives multiple votes across multiple social news voting sites, the higher quality the site.
Categorization : Like Co Citation, categorization can help define the subject of a site, therefore better helping the engine address searcher intent.