I have used social bookmarking primarily to keep track of places on the web that I want to visit later or keep track of.
The option to share those links with other people is good, as long as you remain fairly organized. Just bookmarking for bookmarking sake is will not be helpful for other people.
The ability to search the bookmarks for keywords and find things you want to share with others is cool too.
While Delicious is a favorite of many, I have been using Furl.net for several years.
Bookmarks and favorites are a great way to store and access frequently used locations on the Internet. Furl is designed to archive (as well as share) anything you read online. You may have tens of bookmarks but thousands of Furl entries. If you read your local newspaper online every morning, bookmark it. If you read several fascinating news articles every week, Furl them. You can Furl items from work and home. You can search for them later from any browser, and share them with friends.
My Furl page is at http://www.furl.net/member/weglarz
Furl allows me to bookmark pages and share what I would like to share and keep private bookmarks I would like to keep private.
Try it out. Add some topics, comments, clip a section, and add keywords.
Let me know how it works for you.