I’m fairly sure I’ve seen an NNTP based imageboard that distributed it’s content through that protocol and different instances had overlap of boards. That’s about the closest match to federated system you’re going to find with this model I think. Interesting concept. Not something I’d want to interact with personally though.

Original WhatsApp was XMPP with phone number for your username. Pretty much what https://quicksy.im/ does now.

WhatsApp today is completely different beast.

It’s been a year or two, but last time I tried it their app worked fine on x86 Android in qemu. Not the most efficient way to run it, but at least it’s isolated from the rest of the system.

Re profiling, I don’t think instances will bother doing that (unless they start running ads). However, they also don’t prevent anyone from building that profile themselves from observable behavior. And creating such database might constitute original work by itself. Now, they don’t get as fine-grained interactions as you would with tracking-infested sites. But they will get the most valuable ones such as active participation.