https://www.theregister.co.uk/2019/01/07/facebook_public_officials_first_amendment_appeal/

In what may prove to be a significant precedent, a US appeals court has ruled that Facebook represents a public forum and the First Amendment on freedom of expression applies.

That means, in America at least, public officials using social media are legally obliged to make their accounts accessible to citizens, and are prohibited from blocking individuals simply because they disagree with them.