Well, the XMPP still works and is decentralized. Is also the biggest network. If that is not enough for you then I'm afraid you won't find any other alternative.
You can set up your own XMPP server or use one of tons of the public ones: http://xmpp.net/
Edit: There's also jabber.org http://www.jabber.org/ which probably will operate as long as XMPP is still relevant.
You can set up your own XMPP server or use one of tons of the public ones: http://xmpp.net/
Edit: There's also jabber.org http://www.jabber.org/ which probably will operate as long as XMPP is still relevant.