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.
Do you have any evidence of this strange claim?
Encrypting a message to lots of recipients in an efficient way has been solved for email (via OpenPGP / GPG) a long time ago.
It is a positive side effect of the hybrid algorithms which combine symmetric with asymmetric algorithms, originally introduced to reduce computation time for classic (single-recipient) asymmetric cryptography.