Users of apps based on Parse used to just be users, now they're Facebook users. Combine the fact that a core part of Parse is storing user data, and Facebook's track record of treating such data, I think you should be able to figure the rest out yourself.

If I'm going to pay a firm to store data for me, I need to completely trust that that firm is going to treat said data with silk gloves. I choose what's done with it, not my backend storage partner.

I'm going to be very surprised if the Facebook make-a-detailed-profile-of-every-person-on-the-planet department is going to keep their hands off of Parse data for the next 3 years.