The tl;dr is Facebook is a big company and its cons reflect that. The company mission and values are incredibly admirable, but have become increasingly difficult to implement as the company has grown.
* You might find yourself working on a project with over a million lines of code, but nobody on your team has written more than 10,000. The need to sit down and figure it out yourself can be pretty stressful in an environment that is pushing you to "make an impact" and be constantly shipping new features.
* The few senior engineers (in my org at least) show little interest in mentoring more junior engineers.
* Sink or swim mentality among engineers, the mega-teams supporting products in the core app can be brutal and unwelcoming.
* Launch decisions are very political - criteria for launching a change is opaque, and feedback from department leads happens far too late in the development process (often in the meeting where a launch decision is being made!). Little has been done to fix these issues despite it being constant feedback for the past year.
* Performance reviews put too much emphasis on immediate personal output rather than collaboration and assisting other teams. This causes teams and individuals to focus inwardly. While it's never been malicious, I've seen this behavior directly harm other team's productivity or metrics.
* Work/life balance is below average on the day-to-day, but oncalls are truly awful.
* Decision-making has become a bit too distributed between orgs and reflects in the overall product.
