It seems that you are more talking about social bookmarking rather than viral marketing. There is a huge difference. Viral marketing isnt something you can just "add" to a website. While social bookmarking buttons can help your website go viral, they more than often will not.
Taken from http://en.wikipedia.org/wiki/Viral_marketing So in lamens terms if you want to create something viral it has to be something that bascially makes people want to tell other people. Viral marketing acts exactly like a virus, it spreads by itself. Thats the intent on viral marketing, the trick is to come up with something that instigates it.
Check out the 101 marketing terms dictionary in my signature. I will pm you the link I am still fairly new here.