DoFollow Bookmarking Services
Social Bookmarking is an extremely effective way of generating links, and visitors to a website. The main purpoose of the links is to pass PageRank (relevance and authority for a particular keyword) through to your site. The problem with most social bookmarking deals is that the majority of social bookmark sites append the 'NoFollow' attribute to the links. The nofollow attribute was introduced by Google in order to reduce 'back link spamming'. If a link marked with NoFollow it will no be followed by search engines and no relevance passed for the search engines. This means that PageRank does not flow into the ranking evaluation, thus the search engine positions of your website will not be affected.
Quality not quantity! - you Need Continuous Link Building with DoFollow Bookmarks with Unique Content
There are certain social bookmarking companies that offer DoFollow bookmark social bookmarking services / packages. These packages are guarenteed to use bookmark sites that DO NOT use the NoFollow attribute. Ie. They generate only DoFollow Bookmarks. These backlinks affect the ranking of your website more so than other bookmarking services.