Social networking websites are the greatest place for online commercial activities. People join these websites to get free traffic to their business website. Some prominent social networking websites include Facebook, Twitter, MySpace and more.
Elgg - Elgg is one of the best open-source platforms to stay in touch with people. It offers blog, networking, feeds and file sharing features. Its' versatility offers nearly everything to develop a full featured website.
Drupal - Drupal is a popular Content Management System (CMS). Thanks to its large community support, there are several Drupal modules available. Applying some programming skill and using these modules, anyone can develop a website to enhance interaction.
XOOPS with Yogurt Extension - XOOPS is a CMS, used to create sites based on your needs. Extend your site by including social networking, forum and much more by adding appropriate modules. Yogurt is XOOPS module to add an interpersonal feature on your site.
Dolphin - Dolphin is developed and supported by Boonex. It is partly-free community building application used by dating sites primarily.
Select the open-source from the list which suites your requirement and customize as per your taste. Your social site is ready to use.