Social Bookmarking takes traditional Internet browser bookmarks (or favorites, as they're sometimes called) to a whole new level.  Sites like del.icio.us and StumbleUpon give users the opportunity to store links to their favorite websites online, as well as providing information about the site they have added to their bookmarks.  The greatest advantage to using social bookmarking services is that rather than the bookmarks residing only on your computer, they reside on a server that the search engines have access to. 

Search engines give value to the links found on social bookmarking sites because it is a true indication of the popularity of the website being bookmarked.  Unlike online directories that may require nothing more than a submission to receive a link, social bookmarking is based on a human being having visited the site and determining that it is useful & relevant.  If they didn't find it worthwhile, they wouldn't be making the effort to add it to their social bookmarks.  Because social bookmarks are often also publicly visible, not only do they have an impact on the link popularity of the page being bookmarked, but they also have great potential to drive interested parties to the site by having them click on the link directly. 

We've all heard the phrase "Birds of a feather flock together...".  That applies to Internet users as well.  If someone with whom I have similar interests finds a website informative or otherwise useful, I may trust that recommendation and follow the link.  If I also find the site appealing, I may also add it to my social bookmarks...someone who finds the link in my social bookmarks might follow it and add it to their social bookmarks...creating a virtually endless cycle of exposure for the site and building great link popularity at the same time.  Yet again, a win-win...