You can find out more about social bookmarking on Wikipedia. Wikipedia article on social bookmarking

For more details about each of these services:

Del.icio.us
To register, go to: http://del.icio.us/register
For more information: http://del.icio.us/about

Digg
To register, go to: http://digg.com/register
For more information: http://digg.com/about

reddit
To register, go to: http://reddit.com/login
For more information: http://reddit.com/help/faq

Facebook
To register, go to: https://register.facebook.com/r.php
For more information: http://www.facebook.com/about.php

StumbleUpon
To register, go to: http://www.stumbleupon.com/sign_up.php
For more information: http://www.stumbleupon.com/about.html