Some social networking sites

http://www.linkedin.com/ Business networking site

http://www.foundercontact.com/ Business networking for Entrepreneurs only

https://www.xing.com/ Business networking "Powering Relationships"

http://www.ning.com/ Create your own social network for anything.'

http://twitter.com
A global community of friends and strangers answering one simple question: What are you doing? Answer on your phone, IM, or right here on the web!'

http://www.haystack.cerado.com/html/haystack_directory.php
Networking Directory-self described 'serious social networking for businesses and associations.'