Well they can if fb include a copy of the session key, encrypted with the public key of the escrow authority, appended to the ciphertext. The crypto is done by the fb app, so it is within their ability.
Big companies are easier to coerce than e.g. the pgp developers. There is no way for you to wrap your own encryption layer around the one used by WhatsApp/etc. You can post pgp messages on those systems but that is something very few will do.
The developers of Signal and similar privacy-oriented apps will probably rather shut down than compromise the security of the app. As long as at least one secure app remains, the policy is pointless. And even then there's other ways to communicate securely. There's no viable way to enforce this.
OWS probably wouldn't comply, but have you noticed how hard it is to install something Apple/Google don't want on their platform? Side loading is feasible but improbable on Android, totally impractical on iOS. In place of well engineered solutions from principled developers there would be apps from collaborators. I'm thinking WhatsApp/Facebook will fold pretty soon. 99% of users won't care, and we'll be back to where we were with PGP: most messages are in the clear, the only encrypted ones are huge red flags for further tracking.
