A note on the new version 2 of del.icio.us: We actually use it extensively at work as a professional tool, to share links with one another (and thus to suggest or assign posts and various other things). However v2 dropped timestamps on all bookmarks -- only dates are provided. This has made it about half as useful. And there's not even an option to turn timestamps back on!
Lesson: If your design assumes people will use your app as a kind of casual toy, you will probably foreclose serious, hard-core use.