alt Hacker NewsDepends if you mean just the technology or using it in the small federated spirit. Google Talk and Facebook Messenger were XMPP all the way through and worked with vanilla XMPP clients. Slack wasn't XMPP but supported it via a gateway until it was dropped.
Not sure how popular the small federation was back then, but I know Mac OS X Server touted an XMPP server and that was a first-class feature of iChat.
> Google Talk and Facebook Messenger were XMPP all the way through and worked with vanilla XMPP clients
I remember this, it was great to connect to absolutely every chat platform with bitlbee and pretend that all my chats were just DMs on some irc server somewhere