Earlier quoted context omitted.
How is this any different from sending an event from an account you authenticated from (like an e-mail, or a Twitter account)?
It removes trusting a 3rd party.
The only trust a 3rd party like Twitter demands in your example is that I trust they won't impersonate my account with a message (since there's no private/public key proof)
But your system doesn't address that either. After all, what's stopping _anyone_ from just saying they're me and sending out messages?
Like 99% of decentralized usecases, this still falls down to trying to cheat the oracle problem. I actually trust Facebook and Twitter more in this situation because they can at least demand verification in the real world, they've got enough social capital that they can overcome the oracle problem in limited contexts.