"we currently don't have any plans to offer other forms of authentication, simply because it would slow down the onboarding experience by at least an order of magnitude."
I think you are overestimating both the time required to fill out a questionnaire and the reluctance of people to do so.
I think I speak for a lot of people when I say that people generally don't want any chance of their online dating lives to intersect with their social media profiles. The peace of mind of knowing that there is no interaction between a dating site and their Facebook newsfeed is enough to justify filling out a few duplicate questions.
To a large portion of their audience, that's a feature rather than a bug. Not so much that their information is opened, but rather that they have assurance the human beings they're meeting have real friends.
I think you are overestimating both the time required to fill out a questionnaire and the reluctance of people to do so.
I think I speak for a lot of people when I say that people generally don't want any chance of their online dating lives to intersect with their social media profiles. The peace of mind of knowing that there is no interaction between a dating site and their Facebook newsfeed is enough to justify filling out a few duplicate questions.