Doing QA at large tech companies is never that simple. You have lots of teams that share code. Imagine a scenario where Team A uses code written by Team B which uses code written by Team C. Team C makes a change to their code that breaks Team B's code but only for the way Team A uses it.
For the people in the back, "Facebook is a multi-billion dollar company." They have 30,000 employees. They could spend the money to do better QA. But it's cheaper to let your end-users do it for free.
It doesn't matter if it's simple or not. We can't hold these 1k+ engineer teams to the same accountability levels as a 3 person team. When, as engineering professionals, are we going to put an end to this? This is completely unacceptable in any other engineering discipline.
One way to combat this is to let other teams register tests in other team's projects. If a test fails, you know it breaks someone's expectations. From there, you work with that team to update both sides.
Just to be devil's advocate: Google is notorious for having separate siloed teams that do not share efficiencies or updates (eg. Hangouts and other messengers). A company like Google and Facebook don't have a good excuse, but we shouldn't be surprised.
Gross negligence?
It is difficult to get a man to understand something, when his salary depends on his not understanding it.