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.
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.