Nofollow, dofollow, none of my business to be honest.
Through social bookmarking indexing become more faster.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.It measures the 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.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.
I found the article about nofollow links in wikipedia. May be it will be interesting for you