I'd say that open-source works best for companies when they don't open the main thing. Meta building React in the open is a good example. The community gets a well-maintained library. Meta gets free testing, code contributions and potential hiring pipeline. When trying to compete with Meta, React gives you virtually no advantage. There's no incentive to leave important features out of the public codebase. Both Meta and the community benefit.

Would it make sense for Meta to open the codebase for facebook.com? Aside from studying/scrutinising the code, the only other thing you'd be able to do with it is to change the logo and try to compete with Facebook.

In this example, it's still probably not enough to disrupt them thanks to the social graph and infrastructure complexity. But you could imagine moments where even Meta gets nervous when anyone can start competing with feature parity from day one.

Over the long-term, it's more likely that Meta would want to keep some features private. It's also less likely that they would get lots of quality contributions back. If you're running a fb.com clone in production, you're likely trying to compete with them on some level. This leads to a weird relationship with the community and limited value for both sides.