Here’s a video that explains why using a site such as del.icio.us can be useful. I think they may have failed to mention that you can mark bookmarks as private on del.icio.us, so it’s not necessary to expose your bookmarks to the world. However, in my case, I only mark a small fraction as private.
I’ve been using del.icio.us for quite some time. After I had been using it for a while, I realized that it had been a long time since I bookmarked something in my browser because I had developed a habit of bookmarking in del.icio.us. Most browsers force you into placing a bookmark into a hierarchical, or directory, structure, but on del.icio.us you can assign as many “tags” as you like to a particular bookmark so you can search for things more easily. del.icio.us also allows you to export your bookmarks so you aren’t at the mercy of a proprietary service.
Another thing that is handy is to subscribe to the del.icio.us feeds of your friends to be automatically notified when they bookmark something that may be of interest.