Could Drupal at
http://drupal.org be something for you? I have a lot of affection for and some experience with Joomla, but I will try to learn and use Drupal for my next project. It is about creating a network for teachers using storytelling, and Drupal has some interesting functionality I´d like to try, like the "Organic Groups" module. There is also the Civic Space distribution of Drupal/CivicCRM. I am not saying Drupal is better than Joomla but if your project is about social networking - and you have the time - you should check it out. There is a lot written about similarities and differences between Joomla/Mambo and Drupal and a rather common idea is that Drupal is more suited for networking. I can´t say that is true because I haven´t tried it - yet.
There is also a much simpler and not so mature system called Elgg at
http://elgg.org It is called a "learning landscape" and built especially for educators, but it should be very useful for anyone wanting to create a space for social networking. It has podcasting features out-of-the-box too, which is interesting for storytellers. If you just need a simple system heavily focused on networking it could be worth checking out.
In my own search for social networking tools on the web, I came across an interesting idea. I have forgotten who wrote it, but it was someone with a lot of experience. The essence of it was: "There are only two indispensable tools for collaborating on the internet - a mailing-list and a wiki". The mailinglist I understand, but a wiki? The last time I tried it, it was too demanding for the average user. Anyone knows more about a user-friendly way of integrating a wiki in Joomla? [update] Just found out that there are tools for integrating both MediaWiki and DokuWiki. Much easier to use than I thought.
The User Homepage and Community Builder combination for Joomla sounds very interesting. I have also found the Community Builders forum very helpful. With some luck I might save myself the trouble of learning a new CMS ;-)
If someone has other ideas or more knowledge and practical experience than I have in this area, please tell us more! I think this is a really valuable subject.