They don't get your texts in the message routing sense, but they absolutely have access to the content and could be shipping it to their servers every 5 seconds or whatever.

I don't think they are probably doing that, but there's not really any way to prevent them from doing that.

Excuse me if I don't believe that - even if that is the case today - that it will be tomorrow.

That said, if you already use Messenger for talking to friends, adding SMS isn't going to make a huge difference.

Are you absolutely sure about that? I'd like to believe you but I can't take it on faith. I completely expect Facebook is giving itself some level of info about your SMS messages unless there is some reason they can't.

Trust is the only reason. I'm sure there are plenty of people who are examining the traffic of Facebook apps and someone would make a sink about it if there was evidence they were lieing about their privacy policy. That seems like an adequate assurance for most people but certainly not all.