Think of it another way: accountability is answering to someone. As the majority shareholder and CEO, who is Zuckerberg accountable to? Nobody, really. He might say "I am accountable to the billions of Facebook users", but of course that's lip service: try scheduling a meeting with him to review his performance and put him on an improvement plan.

He has arranged the company so that he is not accountable to anybody, so him saying he is accountable is either a lie, or a misuse of that term.