Let's give Facebook full control over worlds communication? And allow them to cut people off from communicating with anyone? I don't think it's great idea.
Yes, there are downsides to federation, but I would argue spam is not inherent to federated communication.
There are many ideas to stop spam like proof of work.
Main reason for spam in email and PSTN is legacy protocols, which were created long time ago and cannot be significantly improved without breaking compatibility.
We really need federated communicator.