Max Kiesler "an award-winning strategic designer and co-founderand principal of Ideacodes.com, a web consultancy in San Francisco" maintains a Most Excellent Compendium of Open Source social bookmarking, filesharing, search and social networking applications.
Affelio "Affelio is open-source social networking software / architecture. It has following features: (1)distributed architecture (2)Internet-wide scalability, (3)Extensivity with opened Affelio API for developers, and (4)high custamizability with skins/templates."
AstroSPACES "AstroSPACES is the world's first open source social networking solution. Coded from scratch, it is highly efficient and very easy to use."
blogBOX "blogBOX is a free and open source social networking system written in PHP. Future versions will be written i Python/Django."
Elgg "Elgg is an open source social networking platform developed for LAMP (Linux, Apache, MySQL, PHP) which encompasses weblogging, file storage, RSS aggregation, personal profiles, FOAF functionality and more."
FlightFeather: Social Networking Platform "FlightFeather's goal is "social networking for everyone". This means that anyone should have a chance to run a popular social networking site -- on minimal hardware, and without wasting bandwidth."
FriendPortal - An Open Source Friendster "An open-source, Friendster-like social networking portal and news site written in PHP. Post and read news plus browse through contacts like you would in Friendster, Orkut, Tribe.net or Ringo with the knowledge that your personal information is safe."
Geek Grep "GeekGrep is a Django based social-networking system designed to get geeks connected with each other. The main feature is a database of geek codes and the ability to search them. See our project web site for a design template of the future site."
Hiitch: The Social Networking Platform "Hiitch is a secure and advanced desktop social networking platform. It allows you to build a focused and private network of communities for your family, friends, company and etc. It gives you total control and freedom for your social networking needs."
iSocial: Social Networking CMS "Social Networking script written in PHP and MySQL. Designed for every kind of communities - can easily create their own social networking website for free with no ads."
Jahnet "The JahNet framework is a Open Source social networking and asset management CMS that is focused on helping digital artists collaborate on a global scale. JahNet allows you to securely share your ideas, images and projects with users around the world."
Manusya "The Manusya application is an opensource social networking application being built on mod_perl, Perl Template Toolkit, Postgresql, Apache and Linux. The manusya_web_core packages are required for the front-end."
Melt: Online Social Networking Software "Melt is "social software" intended for NGOs to build online social networks, where people can announce events, create groups to organise those events, and add resources (files and web links) to support organisers. Everything can be tagged and linked."
OpenPNE "OpenPNE is a Social Networking Service Engine written in PHP. It has many features(friend control,friend invitation,diary,blog feeds,message box,etc)."
Openpublic "OpenPublic is an interest group social networking and collaboration platform. It provides a solution for mutual interest and special interest groups and membership based organizations wishing to create a knowledge network around their interests."
OpenVZ "OpenVZ is an open source social networking system."
Phpizabi "PHPizabi is one of the most powerful social networking platforms on the planet."
PHP-Spacester "PHP-Spacester is a social networking script such as Myspace and friendster. It is a fork of astrospaces. It will feature the XDNS system (Xotmid Distributed Network System) which is a leaf-hub connection thus allowing anyone to run a leaf and connect to."
Pihook: Social Networking System "Open source social networking system."
Tag Me "Tag Me is a social networking application that allows people to send information about themselves via bluetooth or by mobile web browser you create an online wml website and create a url barcode that holds the link to your online profile."
The Apple Orchard "The Apple Orchard is a multi-user, open source social networking web application with the ability for users to upload photos and videos, write a blog, have comments, personalise their page layout and appearance and sort multimedia by tags."
The Appleseed Project "Appleseed is (augmented) social networking software, ie Friendster, only distributed. Sites running Appleseed will interoperate, and form the 'Appleseed Social Network.' Development is focused on privacy and security, as well as ease of configuration."
Virtual Learning Commons "The Virtual Learning Commons software combines a web based content management system, academic tools and social networking to create a website. Can be used by groups to create web based content within an integrated social networking environment."
WorldSpace WorldSpace is a user-extensible shared virtual environment, aimed at being a next-generation social networking system.
Yogurt: Social Network "This is a Social Network module for xoops CMS. You have seen Facebook, Orkut, Myspace , try Yogurt for Xoops!"