Facebook is the best.

http://www.citeulike.org/
http://www.mister-wong.com/
http://www.folkd.com/
http://www.connotea.org/
http://blinklist.com/
http://digg.com/
http://faves.com/
http://bookmarks.yahoo.com/
http://www.stumbleupon.com/
http://www.delicious.com/
http://www.diigo.com/
http://buzzflash.net/
http://www.bizsugar.com/
http://www.bibsonomy.org/
Google.com/bookmarks
http://netvouz.com/

i am watching the answers and wondering if you guys don't know the difference between social networking and social bookmarking websites. all you mentioned are the social bookmarking websites. Top social networking websites are

Facebook
twitter
Google +
Myspace

If you guys don't know the difference between social networking and social bookmarking websites. all you mentioned are the social bookmarking websites.

Digg , Delicious , Reddit , Stumbleupon, squidoo

Digg.com
Delicious.com
Redit.com
stubmpleupon.com

Digg, Delicious, Reddit,stubmpleupon, folkd,squidoo are some social networking sites.

I think :-
1. Facebook
2. Twitter
3. LinkedIn
4. MySpace
5. Google plus

Facebook,Twitter,Digg,Delicious and Stumpleupon..!!

Digg.com
Delicious.com
Redit.com
stubmpleupon.com

Facebook,Twitter,Digg,Delicious and Google+

Digg.com
Delicious.com
Redit.com
stubmpleupon.com

facebook
twitter
youtube
digg
stumbleupon
fence repair

Facebook
Twitter
Digg
Pinterest
Tumblr

The Top five Social Networking Sites are:
1. Facebook.
2. Twitter.
3. Linkedin.
4. Delicious.
5. Reddit.