In fact, Facebook Messenger's implementation of Signal has very questionable security right out of the box, because if one party "reports" an encrypted conversation, the whole thing is decrypted and sent to facebook support staff.
The Signal Protocol provides end-to-end encryption so you don't have to trust the intermediate parties/servers involved in relaying the message (e.g. you don't have to trust Facebook's servers), and to protect against the other person reporting and revealing your conversation to someone else, the Signal Protocol provides message repudiation [1], which effectively gives the sender plausible deniability because the receiving party cannot prove to a third party that a message came from you.
[1] https://en.wikipedia.org/wiki/Signal_Protocol#Properties
See https://github.com/WhisperSystems/Signal-Android/issues/1181
Let's not make the perfect the enemy of the good.
- Signal code: https://github.com/whispersystems/
Telegram has had known flaws, which have been discussed in part here:
- Telegram protocol defeated. Authors are going to modify crypto-algorithm https://news.ycombinator.com/item?id=6948742
- A Crypto Challenge For The Telegram Developers https://news.ycombinator.com/item?id=6936539
- Telegram (initial discussion) https://news.ycombinator.com/item?id=6913456
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