Sorry for being flippant. I'm just having a hard time imagining how I could post something that you could read that's somehow unreadable to them.
If "them" = Facebook or Twitter... it's true, they could not read the text.
But if "them" = the service provider of the encrypted messages, well, I suspect that they can decrypt from their description. Which means that they can be compelled to decrypt, or that a member of staff could access the decrypted message... in which case we only have an illusion of privacy because it only gives us privacy from some parties and not others.