If they have the ability to decrypt data on their server anytime they want then this whole thing seems utterly pointless. I sincerely hope that's not the case.
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.