While I can’t comment on what may be happening within Engineering, as someone who worked at Facebook for many years, gender bias has been a problem at the company. While it may or may not be widespread, HR pays lip service to these claims, with no action taken. It is often a more subtle form of gender bias that results in women simply leaving the company rather than fighting an uphill and demoralizing battle.
My manager consistently favored male team members and overlooked their sometimes significant performance issues that were not only acknowledged by him, but also shared with him by many employees. I, and other women on the team (most have now left the company in part due to this manager), were treated to frequent criticism and scrutiny, and held accountable for perceived issues even when feedback provided by others was positive. I felt that I had no choice but to leave because this individual blocked career growth opportunities and HR did not address the concerns.