It's impossible to get reliably statistics in a federated system. (Which of course is the entire point.) but looking at the little data I have from people sending me bug reports over XMPP at seems that a lot of Conversations users are running their own servers. The problem is not that the extensions are not implemented (Both prosody and ejabberd in fact support all the extensions Conversations does.) but that a lot of public servers out there are second class services for their operators and are not properly maintained. In that case you can either contact them and ask them to upgrade or ask yourself if you want to have an XMPP account on a provider who doesn't maintain their servers.