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.
Not true, see my reply above. Users on different StatusNet instances can subscribe, reply, and fave each other.