Social bookmarking is one of many ways to locate, classify, rank, and share content. In this approach, users share links to favorite resources and websites with one another instead of relying on search engines. Popular websites dedicated to social bookmarking, such as Pinterest, Reddit, and StumbleUpon provide users with places to store, categorize, annotate, and share favorite web pages, files, and images. In an enterprise setting, bookmarks can also point to files on shared directories. 
http://www.apqc.org/knowledge-base/documents/social-bookmarking-and-tagging-overview-tools-used-knowledge-management