With all due respect to the open source Twitter clones, Facebook didn't beat MySpace by being open source. It beat MySpace by being an exclusive, opinionated, clutter-free alternative to MySpace, and later on a developer platform.
Status.net is the Free software that power Identi.ca - Identi.ca is the showcase instance of Status.net and the biggest to date... But there are many others : http://status.net/wiki/Sites - And you can deploy your own !
No, they're centralized like Twitter. StatusNet is like the Wordpress of microblogging. Each Wordpress installation is its own isolated island. It's the same with StatusNet.