Don't have a Facebook account. Don't want a Facebook account. Even if I did, I wouldn't want it anywhere near anything related to my healthcare. Requiring a Facebook account to sign up was a huge mistake.
We don't get posting rights to your Facebook account. We can't nor will we ever share anything to Facebook. We use FB to verify identity and help make it easier for you to invite friends and family, the people with whom you're already sharing your health info. Everything stays private within Prime.
In other words, personally speaking, it's not you that I don't trust, it's FB. I'd go out on a limb to say that's probably what the grandparent poster meant, too. I know, I know, blah, blah, blah, but I don't want to do the mental exercise of walking through the scenarios. "Okay, Prime has convinced me it's trustworthy, but how could FB hose me? Well, they could...". It's easier to just not use the app. And I want to use the app.