Here is the answer to your few questions.
Q: what is social bookmarking?
A: Social bookmarking is an online service that allows users to add, annotate, edit, and share bookmarks of web documents. Many online bookmark management services have launched since 1996; Delicious, founded in 2003, popularized the terms "social bookmarking" and "tagging".
Source: Wikipedia.
Q: why social book marking is so important?
A: Social bookmarking is very important because many people still use SBM sites and resources that become popular on one often grow enormously, going viral coz the resource is organized by people and not by any search engines. With the help of bookmarking, people can easily create backlink for websites. It helps to achieve a higher position in the SERP.
