XMPP is reasonably specifically a protocol for Instant Messaging. You can theoretically do whatever you want with it, and you can theoretically turn it into a mechanism for delivering social-networking style asynchronous messaging, but it doesn't necessarily bring a lot of value, and it makes a certain amount of fundamental architecture decisions that aren't necessarily the best for a social network.

I am interpreting your "recent open messaging alternatives" in the social network framework because my perception is the major ones you might be thinking about are those things, like Mastodon and such. I'm not aware of hot new open IM protocols that are just "old school" IM.