What operating system does Facebook use?

I'm curious to know, what is the operating system that Facebook employs for their platform? Is it a proprietary system developed in-house, or do they utilize a widely-used and recognized operating system like Linux or Windows? Given the scale and complexity of Facebook's operations, I'm particularly interested in understanding how they ensure the stability, security, and efficiency of their system. Do they have a dedicated team responsible for maintaining and updating their operating system, or do they rely on third-party vendors for support? I'm eager to learn more about the technological infrastructure that supports one of the world's largest social media platforms.