How good is Mark Zuckerberg with competitive programming?

If he weren't good, I can't think who would be better. He built his startup from A - Z. Now, he's a CEO and focusing on his business. Will he forget how to program software? Probably, No. just like asking yourself. Will you forget how to program if you don't deal with technical stuff, but still deal with tech stuff day to day including how to train people, how to grow and so on. Even if he forgets how to program, he can quickly become a man and become better than most of us.

We should ask him 😆

This should be a global question especially when every company asks competitive programming in interviews, we need some real facts. I think he may be good at this because he developed code for the Facebook interface(at least the initial one). Am I right?

Probably not as good anymore. He is a business person, not a programmer. He started of as a programmer but that's not what keeps the business running - he hires programmers that can do competitive programming.