The problem is, that it appears that buy buying a facebook advert you're essentially buying the ability for facebook to refuse to scrutinize you. The argument he's actually trying to make is:
>Should we block all political ads? Google, YouTube and most internet platforms run these same ads, most cable networks run these same ads, and of course national broadcasters are required by law to run them by FCC regulations.
But that's a lie. People aren't criticising Facebook for running the same ads that run on CNN. They're criticising facebook for running ads that CNN refuses to run. But instead, he completely fails to engage with the fact that cable networks have a different, coherent policy. Whilst Facebook's policy is that they're both a free speech platform, but also that they spend more money on getting harmful content off their platform than any other company in the world. That's the reason it's confusing - because the criticism of facebook isn't that its adhering to free speech principles, it's that its selectively using free speech as a defence when it suits them.
Per the First Amendment, Zuck can do whatever he likes with his platform. He can refuse to broadcast whatever he wants, just like CNN and other news networks. But just as important, he is free to NOT refuse to broadcast whatever he pleases. There are no legal consequences for that (outside of the narrow exceptions in Brandenburg v Ohio).
The argument that he is making is that he wants Facebook to be a platform that upholds the ideal of free speech. Whether or not Facebook follows through and actually practices this philosophy equally, remains to be seen.