Basic information that why social bookmarking important for seo :
1 Drive free and targeted traffic to your website or blog - As there are hundreds of social bookmarking sites on the Internet with a smart social bookmarking campaign can lead to thousands of people to your website. Better yet, most of this traffic is very targeted to your website and the topic of your niche.
2 Improves your sites Pagerank and number of links from SB sites - You can tell that by looking PageRank of most social bookmarking sites. The most popular have Page Rank of 4 or more, up to 8. Links to websites, these are very popular and well-organized source for links to their websites that will give you a good ranking.
3 Boost the popularity of the website or the blog - Social bookmarking sites are edited by real people searching for these websites, check the content and rank up or down as they like. This means that your link, mail, or content become very popular on this site, your link or content may be viral – people will rank well, probably they will share on some of their blogs or social networking sites.