Though, with that said, with the introduction of Joomlaworks K2 (CCK) serving as a feature-rich content management system and the soon-to-be released Joomla Anahita Social Engine, there may be little reason to extend beyond the scope of native Joomla.
Not exactly. Elgg doesn't provide CCK-like features and is not for content management in the usual sense. Coupled with Joomla, Elgg offers strong but simple social/collaborative features to complement Joomla's strengths as a flexible content management and application platform. Anahita may be too big for many users with more modest or budget-sensitive needs. Will Anahita have a free or low-cost version? IIRC, it is more of a high-end enterprise-oriented, commercial project.
To minimize segregation of data and content, the best integrated social platform is probably Drupal or Joomla with a native social/community extension. (CCK and general content management is better in Drupal as well, among other things; the problem is the UI and learning curve involved in developing and maintaining Drupal sites.) The downside of Joomla+Elgg or Joomla+anything is it's just a link for user authentication. For anything more, you need to develop and support it yourself.
I would break it down this way today:
Joomla+Community Builder: flexible, widely used, free solution with a fairly convoluted, non-optimal UI and no third-party templates to speak of. Templating is rough, and CB is not a designer's dream. The default layout and views are based on a MySpace concept, and it predates web 2.0 thinking, so a lot of that stuff will have a tacked on feel. CB does not provide any common areas for actual social interaction; it just gives you custom user fields and profile pages with a lot of plugins that add to the profiles. A collaboration with the highly under-developed GroupJive extension is underway, but by itself CB does not offer any group functions. It is not an out-of-the box solution if you care about originality and design.
Joomla+JomSocial: Flexible, popular, ~$100 (USD) solution with a good UI and decent templating options. They default layout and views are based on a Facebook concept. It is pretty quick and easy to set up and have looking good out of the box. Quite solidly Web 2.0 with grood group features and integration with Azrul's comment and blogging extensions.
Joomla+Joomunity: Free. Currently in Beta 3 (January 2009) Development seems slow and their own site has never worked very well. Facebook Connect system has worked well ahead of others. Seems promising and pretty but under-supported. Design concept seems similar to JomSocial, very Facebooky.
Elgg: After a rocky start in the fall of 2008, development and community support is strong for Elgg now, currently moving toward a 1.6 release. (1.2 was released in December 2009. For several years prior, Elgg 0.x was developed, conceptually a precursor but totally separate with no migration path to 1.x.) Elgg is all social network and only social networking. It has a Facebook 2.0 flavor influenced by Twitter, but (showing its educational roots) it is more oriented toward collaboration with user blogs, group blogs, and group forums. Organizationally--as you can see on the Elgg Community site--it is not a good solution for large sites that do not have a strong curator team, especially since it is weak on moderation functions. If text information and especially file sharing (where version history is critical) accumulates at a high rate, Elgg becomes soup--although for the purposes of fostering anarchic, hands-off growth in an open source community, this is the typical model. Elgg has no backend to speak of, uses an MVC system, and is modified via plugins that supplement and/or override both functionality and design. (Themes are plugins.) If you've done any significant under-the-hood customization with Drupal or Wordpress, Elgg will probably seem more accessible and somewhere in between for its learning curve.
Drupal: Strong and well supported but challenged with a rough UI and brick wall learning curve, you can really build anything (and do it right) with Drupal, given enough time. Since Drupal was originally built with a hybrid concept of content management and collaboration, it was conceptually ahead of the curve for web 2.0 and social publishing. It gives you a good framework with well-integrated extensions/modules that play well together, allowing you to build highly customized sites with the social features you want and solid content management, custom workflow and access roles and rules. It's just not an out-of-the box solution for people who wannt to build something fast but have not prior experience with Drupal and maybe nothing similar.
SEO strengths and weaknesses of all four are important, but I don't know much about them except that Drupal is great for SEO. Elgg was pretty bad as of the 1.2 release, and its internal search feature was worse than Drupal and Joomla's. (They're all sub-optimal in their own search features.)
On the web since 1995. 495 posts in the old Mambo forum. Under the hood with Wordpress, Joomla, and Drupal since 2006. Many more platforms before and since. http://www.newlocalmedia.com http://www.twitter.com/newlocalmedia