If Facebook had to add a backdoor, so would all the competitors. I don't think anyone was going to switch any time soon were they to add the functionality. Any service worth their salt would switch to "oppressive government mode", either disabling access or ignoring the law and fighting the block when it eventually occurs.

Facebook has been under pressure for many years to allow government agencies to decrypt messages. I believe that their "no" is also an ideological one, because I can't imagine any of the staff being willing to disable security for their customers for government oversight.