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Surprised this impacts Whatsapp as well which seemed to store everything encrypted on your device...

It needs to download it from the CDN first. The actual implementation is your media are stored encrypted on a CDN for a certain amount of time then you receive the keys as an encrypted message.

Likewise. If it's a well-thought out encrypted blob store, fine...

But given that Facebook, Instagram, and WhatsApp would seem to have vastly different caching and security requirements - its hard to believe.

Is any of this architecture publicly documented?

I think Zuck himself said recently they're moving to encrypt everything and merge infrastructures for single login across properties (which a lot of us think is them sneakily trying to make their web properties a lot harder to split up in an anti-trust case).

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