We want to get people meeting each other offline as quickly as possible (not filling our laborious questionnaires) and Facebook really helps to lower that friction.
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.
Surely having an option to manually fill in those fields (with the addition of asking email/password/name) instead of pulling from the Facebook API is within reason.