[nickpsecurity]

I'm going to assume that on top of the complexity of programming itself. The reason is that I previously read that Facebook lets new hires work directly on the live site. They tend to just push code out there. They also do it in tools like PHP where it's harder to automate QA since no language-level annotations that make that easier. The combination of tooling that makes problems easy plus nothing stopping inexperienced people getting bad code out there means Facebook has higher-than-average risk of problems happening.

Good news is that their people are smart. Makes up for it a bit. Inexperience will still bite them, though.