I recently looked into the availability of social tools for popular cms systems and found some promising updates related to some major open source content management systems.
Best Open Source CMS Awards
1. Drupal
2. Joomla!
3. CMS Made Simple
http://www.packtpub.com/award
Best Open Source Social Networking Content Management System
Wordpress Wins
http://www.packtpub.com/article/wordpress-wins-best-open-source-social-networking-cms
Some additional advances
Drupal - Drupal 6 Social Network Distribution (a Ning like project)
http://groups.drupal.org/node/10468
Drupal - Social Networking Tools Group
http://groups.drupal.org/social-networking-sites
Joomla - Groupjive Social Networking Joomla Component
http://www.groupjive.org/
Elgg - Open Source Social Platform
http://elgg.org/
I'd also like to think that there are some developing third party platforms that will plan to support cms systems with API's or even simple profile and community functions that are script / cookie based.