I didn't like FB well before it became fashionable, but I don't understand your logic here: what is that exactly that you can't entrust WhatsApp to send, but you can trust Apple, or your mobile carrier?
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.
Given that I don't see why more of us Americans aren't using WhatsApp! :)
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.