Give me a benefit, outside of the social aspect, to use your site
Del.icio.us organized bookmarks for me by tags/categories that I create myself on-the-fly. I want to remember places I’ve cruised on the Web, so this is useful to me.
Loosen the constraints
One of the coolest things about del.icio.us has been sharing links with friends and acquaintances. But I never had to add anyone to my trusted buddy list. And del.icio.us didn’t even officially add their only social tag (for:, which sends a link to another user) until last July. They let the social aspects build on their own first.
Know thy audience
Del.icio.us shows they know their users by adding features as needed. And they seem to get that we’re techies, too, which might be one reason those loose constraints work so well.
To me, this is a whole lot better than a site that lets me set music to annoyingly play when the page loads.