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. These include Organic groups (for building communities or groups), MySite (module for MyPage or MyYahoo type functionality), Panels (module for creating more flexible layouts) and Views (module for creating flexible lists of content) -- all modules that are necessary for building such social publishing applications.
Meanwhile, some of the problem related to 3rd-party modules could be reduced with the new firm Acquia announcing a commercial Drupal version which will include support for many of these 3rd party modules. Even though Acquia has received a bit of publicity (and VC funding), the first release of "Carbon" (its commercial offering) won't be released until the second half of this year.
So if you are planning to employ Drupal for your social software project, pay special attention to the modules that are required and test them thoroughly on Drupal. It is quite possible that you do not need any of these 3rd party modules because Drupal also includes many modules like Blog and Forums in its core. But if you do need external modules like these, your best bet would probably be to go for Drupal 5.x (and the same holds good for those looking to upgrade from 5.x to 6.x as well). As we continue to research social software in the enterprise, look for more details in these pages on Drupal and competing platforms.