Joomla is a great platform, but way more generic than Dolphin. It's more content-centric and lacks lots of social-interaction features that Dolphin has. There're different extensions/plugins for Joomla, but again, you'd spend a lot of time and effort to build a full-featured social network based on Joomla. That's BEFORE you get to the Flash apps/servers part.

I see Joomla as a more technically-challenging platform for content-centric sites, like corporate sites, news sites, etc. For social networks see more you need advanced profiles, media sharing, friends, chats, etc, etc. All that comes "out-of-the-box" in Dolphin.