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.
I really like the concept BTW!
Thanks!
EDIT: Seriously, I appreciate the desire to get your viral coefficients up and everything, but this is something that is actually important and can ruin people's lives; don't take it lightly.
Eh, it's not a disastrous problem -- this seems like it would be something you can change without any serious reworking of your central offering. You could also have a one-time, optional Facebook connection where the patient could authorize you to grab their contacts, and then have no further contact with FB.