Facebook operates in California. It’s users are - by Facebook provided user agreement- bound in their relationship with Facebook by California laws.
Now, the constitution of California provides positive right of speech to it’s citizens: Every person may freely speak, write and publish his or her sentiments on all subjects, being responsible for the abuse of this right. See this dot here? no exception for Zuckerbergs. Or your car.
Then, there is courts’ interpretation of this right. It was as following (by memory): if you created a public forum, then you can’t remove people from it for their speech contents only . That’s why California malls can’t remove beggars soliciting money (you car is ok, it’s not a public forum).
Now, as in now, California becomes a one-party state (feels like home, sometimes). The same Supreme Court (but with nowadays judges) decides: is Facebook a public forum? You can guess the answer.
So you agree. I can carve a message into the side your car, and there’s nothing you can do about it.
Nice. I’ve got my knife. Let’s meet up! Afterwards, we’ll swing by the television station and make them broadcast us live, free of charge or control, a six hour lecture about our novel interpretation constitutional law.
So your argument on why facebook with 4 billion users should ban whatever it likes on its platform is that a car that can fit more a dozen people shouldn't be vandalized.
Then they came for the internet registrars, I did not speak out because I didn't have a website.
Then they came for the payment processors, I did not speak out because I didn't have an online business.
When they came for my bank account there was no where left to speak.