You obviously know more about this than I do, so it's likely that you're right and I'm wrong. But: To me, it seems unlikely that encrypting a message to thousands of recipients (a fairly normal use case for FB) isn't going to massively increase the message size. It sounds like you're going to have to include encrypted key data for each of those recipients, and while I can believe there might be surprisingly efficient ways of doing that, I have a hard time believing it's not going to lead to extremely large status update messages.