Evidently Facebook themselves don't agree with you, since their "Secret Conversations" feature uses Signal's protocol (many other systems also have equivalent features built out of Signal Protocol, Skype, Google Chat, XMPP ... it's a sort of trend)
In terms of how Signal compares to something like Facebook Messenger using HTTPS that's an actual technical question that's worth talking about (whereas "Oh no, Moxie Marlinspike is a mean person who hates F-Droid" is basically just noise)
HTTPS gives us two ingredients, both relevant here, TLS and HTTP. TLS is intended to deliver privacy and data integrity between two parties, it does a pretty good job at that, but it's very common to have configurations which do not result in Forward Secrecy. This means messages that couldn't be read by any potential adversary today could become readable tomorrow - long after both parties don't need them any more - if the adversary obtains a long term key. Your web browser might, possibly, tell you about this risk if you ask, but it definitely won't flag it in the main UI. Next, Facebook is authenticated to you by its proof of possession of a Private Key corresponding to the Public Key in a certificate from a Trusted Third Party CA. An adversary could corrupt this CA, but hopefully that's difficult.
Now, why does HTTP even matter? Well, although TLS _can_ authenticate both parties, on the Web today we rarely do that. Instead the web server is authenticated using TLS but the client (a Facebook user) has some crummy HTTP layer authentication, maybe a password like "1LvUrDog" filled into an HTML form field.
So, between those two cracks, if the Secret Police either steal your brilliant canine-related Facebook password or get Facebook to give them a long term key, they can read all your Facebook Messenger chat, eavesdropping on it indefinitely.
Signal doesn't use passwords. Your device has randomly picked a Private Key, but unlike Facebook you don't have a certificate from a CA, instead you can compare the associated Public Key on your device with that shown for another participant on their phone, if they don't match there's a Man in the Middle. So immediately that's an improvement, no password guessing.
Signal also has Forward Secrecy. In fact each message sent and received changes the keys used for future messages. As a result an adversary can only eavesdrop by actively impersonating one of the participants. In a two person conversation that's often likely to become obvious pretty quickly whereas passive eavesdropping is undetectable.