So yes, Facebook intercepting Snapchat messages is completely inexcusable.
But at the same time, Snapchat explicitly designed their app to allow message text to be intercepted. (Pictures are end-to-end encrypted)
What was the rationale for this decision, I wonder?
Though as I look closer, it appears that Snapchat has been doing cert pinning since 2015 at the latest.
So, if Facebook was intercepting Snapchat traffic in 2016... what exactly were they successfully seeing? https://github.com/magicguru/SnapchatCertPinning