Joomla has free extension
Community Builder and en extra ordinary but premium one
JoomSocial.
Main difference between Joomla and Wordpress is the way they classified their Extensions:
1. WordPress has Themes Joomla has Templates. Basically it's the same thing, basically. Both are comprised
of basic HTML and CSS and then you use WP or Joomla platform to build an otput to the page. There is a pretty much differences when it comes to coding one or another, but in both case you need good knowledge of PHP too. Some of the developers have taken it to the higher level and made Frameworks for each of them. But you can say generally that both Platforms are basically a kind of Frameworks too.
Wordpress has the loop and other constructs to get all the content for the page, Joomla outlines all the available module positions.
Joomla modules are similar to WP widgets and Sidebars (in WP sidebar is not just something that displays on a side of the page ...). But I must admit that Joomla Modules are more flexible then WP widgets and Sidebars, but on the other hand much more demanding. But you should never forget those two are made for different purposes! ...Ther is no enough space in a single Forum Posts to explain it in any more detailed way.
2. Beside Themes all the other WP extensions are called plugin/s. Joomla has Componets, Modules and Plugins. Joomla plugins are something absolutely different then WP plugins. Joomla Plugins are literally plugins. It means just an extra add-on to the Module or a Component. Component can have more Modules included, but Modules could be standalone extension too. Joomla extension system is much, much more complex then WP it is the main thing that attracted me to Joomla in a first place, much more freedom in creating different kinds of Websites. Components could be a separate Platform for it's self, like k2 or ZOO ...
But on the end I've chosen WP ...Joomla is much more demanding about everything, from Hosting environment to all other aspects ...it is made for a huge projects, Portals ...WP is much more appropriate for the most of the common Internet Users.
Both are pretty poor about User Management ...basically you don't even have an option to upload user avatar?! ...and need Components or plugins to create good User Management System.
I know this is to General ...after all this just a Forum Post, not a tutorial ...