del.icio.us is a web-based bookmarking site. Delicious users save their bookmarks on the website so they can access them from any browser. A website that has many bookmarking it is generally popular and will get more traffic. You can use del.icio.us bookmarks to help improve your SEO campaign. The public social component of del.icio.us makes it a great tool to make your website stand out form your competition. 

Here's a video explaining how to use del.icio.us and why is it SOCIAL: