I used xmpp, and also Gtalk, both of them. Talking both between xmpp clients and Gtalk users.
XMPP was a mess. Messages often didn't arrive if target was on a different server. And different clients had different encryption and encoding standards, so even if it arrived it wasn't always readable (or just completely ignored by the client). Images seldom worked across different clients.
The only way to make it work reliable was if everyone was on the same server with the same client with the same client version. That is, if the server itself hadn't crashed.
The reason Google stopped the link was because while it had very little of legit messages on the platform, it had most of the spam and trolling. XMPP servers were the "soft underbelly" of Google talk, with the small server admins not having the resources to deal with bad actors even remotely as good as Google.
You know Facebook has released a ton as open source, right?
Edit: ever heard of React? Facebook. Pytorch? Facebook. GraphQL? Facebook. Llama? Facebook. Plus a ton of more niche stuff. Zstd, relay, fresco, hydra, redex, React native..
Edit2: https://github.com/facebook