They control the client, so they can do whatever they want. They can take the plain text, encrypt it with my key, encrypt it with their key, catenate the two,

They control the client, so they can do whatever they want. They can take the plain text, encrypt it with my key, encrypt it with their key, catenate the two, send to FB, split off their "copy" and decrypt it to do whatever with, and send my "copy" on to the recipient. e2e isn't a tech issue, it's a trust issue. Do you* trust FB? * You in general.