Social bookmarking began long ago and some of us who are old enough remember card catalogs. Libraries used this social bookmarking system to standardize a way of classifying books by author, title, or Dewey Decimal System.
Social Bookmarking on the web is a 2.0 platform created to allow groups to classify, store, and retrieve internet resources.
What is lacking is a standardized classification system like Mr. Dewey’s decimals. 
Hence, the cons already mentioned.
Another con not mentioned is when the link goes dead. Would be like a library book that gets removed. If the card remains in the catalog, many seekers may be led to a vacant shelf. And similarly with the web-based system.
GOOD NEWS though! The semantic web will cure this ailment of dying links. Rather than tying knots to link pages with bookmarks, Semantic Web will tie a pretty bow which can be removed, updated, and re-tied when necessary (or at least we hope so).
I suspect ‘favoriting’ pages will become more useful so that others can search my tagged favorites which will only contain those favorited items that still exist. Make sense?
