Social bookmarking is helpful in SEO (Search engine optimization) purpose in various ways and is one of the most used and very popular method in SEO industry. As the link we add online is easily indexed by google as being placed on a high pr website provide us some good quality backlinks which are indexed very fast along with helps in indexing our webpage.
As both the dofollow and nofollow links are necessary in backlinks process and both have their own importance. Your website should have both dofollow and nofollow links so as the link building should look natural , therefore if the social bookmarking site at which you are dropping your link has links as nofollow they should also be used as they have their own importance.
Twitter is one of the most popular and widely used websites in the world, in the recent PR update twitter is now a PR 10 websites which means placing a link in twitter will provide you a good and high quality link which will surely help you in increase in websites rankings or can also be used for indexing your webpages.
Delicious is another social bookmarking website which is also widely used and provides do follow links from the bookmarks. This website is having a rank of about 2000 in us and is getting very popular day by day.
Digg is another social bookmarking site widely used for social bookmarking and get some backlinks the alexa rank of digg is under 1000 and is a high PR authority website to drop your link. Most of the content links at digg will be technology based however other kind of links are also found there.
Clipmarks is one of the website where you can add some of your important content of website like a paragraph or something and save it there and rest assured about your data being stored at a safe place which can be accessed anytime afterwards. It also allows you to add the content like images, videos or simply text.
Newsvine is one of the most popular website for creating sub pages, posting your articles , social bookmarking your website. Its alexa rank is within 1000 and one of the most used pages people use to add their content. It is also having a community wher you can share your views over specific topics and get answers to your questions along with some good SEO links for your website.
That was the list of top 5 social bookmarking websites which you might find useful for SEO. if you have any questions feel free to comment and i will be happy to answer.