In general you cannot be certain of anything in today's world with regards to security. You have to trust someone. Actually, you have to trust a lot of people: hardware vendors, compiler vendors, OS vendors, and application software vendors. Any one of these could, if they chose to, easily introduce a back-door into your "secure" system. The only way to be absolutely certain that no one has done this is to build your own hardware from discrete components and write all your own code from the bare metal on up, and even then you have to trust yourself not to have made a single mistake in a realm where even experts regularly do.
Personally, I don't trust FB, but for a non-technical person there aren't any clearly superior alternatives.
Signal's iOS app is really nothing to write home about (imho) and its desktop app is often downright unusable.
I have not used Wire beyond a test install but it indeed looked promising. In a sense it's even better than Telegram that allows you to just use usernames but you have to add a phone number anyway (also if the other Telegram user by any chance has your phone number in their contacts then your identity is revealed anyway).
But I'd say that Telegram's apps (mobile and desktop both) are superior too all the major IM apps around. I wish they could be more forthcoming about openness.
Personally, I don't trust FB, but for a non-technical person there aren't any clearly superior alternatives.