You speak kind of to the upside to having FB auth. It makes it easy to try services like this without committing to a password. If I'm on my phone or playing around with Show HN apps like this I'm probably not going to bother giving an app a unique and new password and I certainly don't want to give it my go-to lazy password. Because of that I feel like things go a bit smoother if they have FB auth. The benefits became clear to me when LivingSocial got hacked - I didn't have to worry since I use FB auth on LivingSocial and they never had my password in the first place.