Pandia Search Engine News tested over 20 social bookmarketing tools to come up with the 5 they feel are the best.

Some of the critical they based it on:

• A decent amount of users is necessary. Otherwise the whole social aspect looses relevance.
• The service has to have quite extensive documentation, FAQ and/or help pages, to guide users.
• More advanced users will want good import and export possibilities. Import is necessary for bringing old bookmarks from your browser or from another online bookmarking service, export is important for backup copies and to ensure that your bookmarks can go with you to another service if you decide to switch.
• A good bookmarking service should have feeds (RSS and/or Atom) of tags or groups of tags.
• I would want my bookmarking service to have a blog so I can be kept up to date about new features, bugs, bug fixes and more.

This is a good article that will benefit your viral marketing (aka linkbait) promotions with social search.