By the same token, a person who happens to use Facebook can also "send messages" to a person on Mastodon. They just won't be FB messages from the FB account.

At no point has anyone said anything useful about the interoperability of either system, let alone how they compare to one another. All we have learned is that everyone is equally free to have a bunch of accounts, whether they be social media accounts or cryptocurrency wallets.

This is why I think Mastodon et all should store their account data and user identity details clientside. That's how to ensure it's easy to switch services without losing your existing network by default.