Facebook's official policy for a while has been, don't use the phrase "culture fit". The interviews previously known as "culture fit" are mostly asking you questions designed to answer things like, do you work hard, are you good at working with other people, are you an asshole. In my experience as an interviewer at Facebook, most people rejected for cultural reasons were either jerks during the interview in some way, or expressed that they weren't really interested in the job. Really nothing to do with ethnic background.