en.wikipedia.org/wiki/Social_bookmarking defines Social bookmarking as a method for Internet users to store, organize, search, and manage bookmarks of web pages on the internet with the help of meta data.

For answers on what, how, why …, you can download this 2 page report from Educause.
Listed below are Top 45 bookmarking sites.

I suggest you to register to each of these manually once and use a bookmark submitter to submit. My choice of submitters are www.socialmarker.com & www.socialposter.com.