Why It's Harder To Encrypt Facebook Messenger Than WhatsApp | Motherboard

it's easier to roll-out encryption by default for WhatsApp because it lives on only one device (you can’t use it on multiple places, except by syncing it on the web),

while Facebook Messenger lives on multiple devices, and people use it both via the web (on not one but two different sites, Facebook.com and Messenger.com) as via the app. That’s the big challenge here: people use Facebook Messenger via those websites, and doing encryption via the browser is notoriously hard.