"The open source package Drupal is one of the few social
publishing platforms built on top of a longstanding Web CM (WCM)
system. It has a strong foundation with a very flexible taxonomy
system which--along with thousands of 3rd-party modules--enables
you to assemble social publishing applications.
"However, these modules could be your biggest problem as well
because many times, module upgrades do not keep pace with Drupal
upgrades. Even though Drupal has released version 6.2, many of the
more popular modules are still on 5.x..."