i have been searching for a while for  instant turnkey php scripts  to start my own social network site. I must say that i am totally impressed with the scripts and these are the best ones i have seen to start a social network. I have listed both open source free ones and commercial ones are listed below:

PHP Fox

http://www.phpfox.com

phpFox is the most popular social network script written in PHP. It is a instant turnkey social network solution and is commercial.

Social Engine

http://www.socialengine.net

Social Engine is a feature rich, social network and community software for websites. It is commercial and comes with 100% unencrypted code.

Elgg

http://www.elgg.org/

Most powerful social networking engine written in PHP. It is available free to download.

Boonex Dolphin

http://boonex.com/dolphin

Buddy Press

http://buddypress.org/

Social networking platform based on WordPress MU. It is FREE to download.

DZOIC

http://dzoic.com/

Dzoic Handshakes professional is a very advanced, comprehensive social network script written in PHP. It is a commercial software.

Pligg

http://www.pligg.com

Pligg is a open source social publishing and bookmarking CMS for your website. It is free script.

Pointter

http://www.pointter.com

Microblogging and social network solution for Websites. It is a free software.

PG Social Networking

http://www.socialbiz.pro

Open source social network software for community websites. It is an instant turnkey social network solution and packed with advanced features. It is commercial script.

WebNetwork

http://www.webscribble.com/products/webnetwork

Webnetwork is a social networking php script. It is commercial.

MyOwnSpace

http://myownspace.sourceforge.net/

MyOwnSpace is a free open source social network software written in PHP.