If you have secrets at all, don't send them through any ActivityPub conversation.

People on Mastodon make this mistake quite often, tagging someone they're talking about, or realising that the person they tagged now receives a copy of their conversation.

This is a massive issue on top of the lack of end to end encryption. Both servers receive plaintext copies of the messages exchanged. I'm sure mastohub.ai is a safe server, but how can you be sure they'll never be bought out or hacked?

If you want to federate and share secrets, try something like Matrix or XMPP. They make it significantly more difficult to read your messages.