This is pretty hard and is usually anecdotal as both companies claim a standard 40 hour week and great WLB (FB does boast about its break fast fix fast culture). I usually use the "interviewer age" as a proxy (not a great one but something). My interviewers at Google have usually been "older" people with families etc where as at FB they are usually the "energetic" ones. Interview questions reflect this too. I found Google ones are usually more testing of experience and are generally well rounded than FB ones (ok let us do leetcode problem # 35, 46 and 82 in an hour).