There were two studies, one that was unofficial (just an engineer looking at commits) that found the quoted result. It looked at gender adjusted by tenure. This

There were two studies, one that was unofficial (just an engineer looking at commits) that found the quoted result. It looked at gender adjusted by tenure. This study didn't have the resources to do that. Then there was a second cover-your-ass study by Facebook HR that found that if you adjust gender by seniority/rank the bias went away. This raised the question of why female engineers were at lower ranks after the same amount of time on staff. It might be explained by prior work experience, or starting rank, but it might not be. From the articles on this it looks like they (fb) don't actually want to answer this question.