The external metadata compiled via user-generated descriptions, tags, headings and categorization is amazingly valuable to search to use of Social Bookmarking, as in the same philosophy as anchored inbound links, descriptive content about a site defined by you of that site who aren’t associated with the marketing or coding of that site, can be extremely powerful in gauging the value and relevance of the content and tags which are being used on that site.

Bookmarks show how a site is perceived, and when these sites allow voting, they also show the engines or whatever classification system which displays voting, how people feel about the quality of the site. Furthermore, sociable bookmarking can introduce a web site to the search motors, as in many cases, people may find and book mark a site or a site’s internal pages before the search engines can find those pages via another form of inbound link.

Monitoring social bookmarking services like Delicious and StumbleUpon help search engines in multiple ways by:

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 frequently not as easily associated to by others, found via bad or absent site navigation or connected to from external web 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 more of an authority than a site with only several bookmarks by the same user.
External Meta Info: Users who bookmark sites tag them with keywords and descriptions which add an honest and fair definition which is done by the public and not the master of the site.
Co-Citation: Social bookmarking sites usually tend to categorize sites and pages based after the tags employed by humans to spell out the site; therefore search algorithms can classify these sites using their peers.
Amount of Votes: Exactly like the amount of bookmarks, the more votes a page will get on Digg or Reddit, the more useful that information usually is. In the event that the same page obtains multiple votes across multiple social news voting sites, the bigger quality the site.
Categorization: Like Co-Citation, categorization can help define the subject of a site, therefore better helping the engine address searcher purpose.
To aid share the prosperity of social bookmark management, I’ve put together a set of social bookmark submitting sites, some of which are incredibly popular and others which are new or somewhat unheard of. But besides targeting the major social bookmarking sites, you will find some diamonds in the hard which are niche focused or treated well by Google and other search engines like yahoo.