Do Follow social book marking sites are the sites that give a free back link for your website. You have to just register/login these sites and then you put your link of site in these sites. In return these sites give you a back link that is very valuable for your site and in this way there
are many chances to increase your site page rank as the page rank of Google depends on your site’s back links. The higher the back links your site has the greater the chances your site has page rank.
All of webmasters use back links to increase their site’s page rank and they use other techniques as well as Do Follow sites to raise their back links. Back links are the back bone of Google page ranking. Here is a huge list of Do Follow Social Book marking sites.