Indeed, and not only are their tests automated, they also rely on production traffic to expose failure cases. Since some problems that are exposed are only applicable at scale. They use canary deployments to slowly ramp up traffic to the new version; rolling back if they detect anomalies.
Maybe you'll find this video interesting: https://youtu.be/2mevf60qm60
The article talks about static analysis, I wonder if they do human code reviews at all?
Any which way we slice this, this is incredible! Sure instagram is not healthcare, transport or banking application - nobody is going to die if the website goes down, it is still an awesome achievement.