Facebook sells the ability to do this to anybody who will pay. This is the core of how they make money. After a period of pretending to search for a solution, Zuckerberg has finally ducked behind the notion of 'free speech' as his excuse for doing nothing. He knows that the only way to stop it would be to stop selling this ability to influence. And he's actually right that it shouldn't be him deciding what are the good and bad uses of this powerful tool.
The blind faith that increased connectivity is a net benefit is what got us here. Seeing how social media is actually increasing depression makes me think the opposite. The sad truth is that most people are too vulnerable to misinformation and the older centralized distribution of information is probably the best for stability.