Apple's iMessage is secured end-to-end, your mobile carrier can't see the message. As soon as the box goes green then any intermediary, including your mobile carrier and your friend's mobile carrier, can read the message and/or make it available to law enforcement.

In a U.S. court of law Apple has stood up to authority and pointed out they can't provide the messages requested as they're locked even to Apple. Facebook on the other hand has acquiesced to U.S. law enforcement. That tells me that even if Facebook has secured end-to-end messaging, which I don't think they've ever claimed, they have backdoors.

I'm not really answering your question, but in the States, there is a lot more trust placed in Apple over Facebook. This isn't that surprising - Apple has a much stauncher stance on privacy than alternatives.

I wish Signal would be more popular, but it's just not. I can't really ask new people I meet to communicate over Signal because it's a burden to get others to install some new application.

I don't know about GPs logic, but my logic is pretty simple. I don't use any infrastructure that Mark Zuckerberg had his fingers on. That was clear to me after the first time I saw him.

Facebook's goal is to extract as much information from you as possible. Apple wants to sell you a new phone.