2) Faster Search Engine Indexing: Most of these Social Bookmarking sites are authoritative sites. The search engine spiders visit these sites so often that your links will be crawled almost immediately!
3) Higher Search Engine Ranking: Getting one-way incoming links is the main factor to higher search engine ranking. With Social Bookmarking, you can get unlimited 'keyword' links to your site. Having more keyword links that are relevant to your site will result in higher ranking.
4) Better PageRank: Having high PR Social Bookmarking sites linked to your site will improve your site's PR. With a higher PR, you can link to your new/existing sites and improve their ranking too! You can even monetize on it by selling links from your site!