he purpose of social bookmarking sites for me is to get a do follow back link to boost the page rank of my articles, get indexed by Google faster, and possibly get a lot of extra traffic.
Some social bookmarking sites have removed the do follow attribute from the links you post, and are therefore fairly useless, unless for some reason you want to share bookmarks with people on a site that nobody uses.
While it's true that Facebook and Twitter have no follow links, they are popular social networking sites, and Google does place some importance on the number of times your link appears or gets "likes" on these sites, plus you may get traffic from your friends and followers there.

