Zuck could be successfully sued if he acted in a way that obviously enriched himself at the expense of other shareholders. The court does not recognise that he owns the entire company, therefor he does not.
And that's probably the right way to look at it in most contexts. But if there were a controversy about whether Zuckerberg is using Facebook to spy on people, it would be a really silly deflection for him to say "I think you'll find that's impossible because Facebook is owned by the American people".