First of all why do we want a social bookmarking site? As the name suggests we need a social bookmarking site to bookmark our favorite pages, sites, articles or any interesting things we find online. We start tagging all these interesting things online but not on our local computer. We can access them later using any other computer or share with friends too.
What is dofollow?
The most important use of a social bookmarking site is that the users keep coming back for any updates on a regular basis. Most of them allow users to have a backlink to their site. But if the quality of the content is not good, users may not come back again thus losing the valuable traffic. This resulted in bookmarking pages to lose their valuable link juice benefit to other pages. This also resulted in losing page rank value etc.
Here, there are primarily two kinds of backlinks allowed to be posted in any content.
In the case of dofollow, the site instructs Google bot to crawl the backlink back to its origin. This way the targeted page gets valuable link juice benefit. But here there is one problem. If the site allows every user to link back to their pages irrespective of the quality of the content, this will greatly reduce the overall benefit to the site. Because backlinks to the bad pages will eat up the reputation of the hosting site. So over the period, social bookmarking sites started recognizing the fact that all the backlinks of the site do not point back to the good pages only but to the bad pages too. They found that ultimately they are losing the value of the site. So they decided to make all backlinks from their site as nofollow so that backlink benefit will not be shared back to other pages. This way they are not losing out any link juice to any other pages.
By using only social bookmarking sites which allow dofollow backlinks one can get the benefit of backlink juice. Here Google spider crawls the backlink pointing to an external site and index that page. This way that external site gets indexed by Google spider thereby allowing it to be ranked in the search engine results.
Another benefit is an increase in the traffic. Link or no link, any page with a unique and quality content always generates a following thus creating traffic. More traffic means better reputation of the site. Ultimately this also converts into a better rate of return from a business point of view.