If you’re talking about decrypting messages encrypted created and read via SSL (what they imply is the case), it’s not possible unless you have the private key, versus the widely available public key.

I doubt it’s lying around in Facebooks repositories but I’ve never worked there so cannot say that is the case with certainty.

This is all assuming they are even using modern SSL and are careful with user data. Unfortunately, not a great track record there for FB.