Zuckerberg is not popular. This limits his power in terms of human influence - he has virtually none. He cannot get others to do his bidding. He cannot convert others to his ideology and way of thinking. He cannot alter people’s mindsets or set their political beliefs. Therefore, in human terms, he is virtually powerless.
Zuckerberg also has limits to his power. He must behave in accordance with the law. Should the government pass a law to break up his monopoly, he would be powerless overnight. Such a precarious source of power that depends on the approval of the legislative and legal bodies is not compatible with the notion of unlimited power.