Facebook Blog
http://blog.facebook.com/

Facebook Help Center
http://www.facebook.com/help/

Facebook Pages Info.
http://www.facebook.com/pages/learn.php

Facebook Statistics
http://www.facebook.com/press/info.php?statistics

Twitter: About
http://twitter.com/about

Twitter 101 for Businesses (best practices, advertising, etc.)
http://business.twitter.com/

Linked In
http://www.linkedin.com/