It's not that simple, and I hate it when it's inconsistent.

For example, even in the US, courts have established that Twitter is a public forum with respect to what politicians (such as Trump) say. But do other people get such protections?

In my country, Romania, the supreme court declared that Facebook is a public space, so if you say something like "the police can blow me" you'll get a letter from the police telling you so show up at the station so that they can fine you (swearing is illegal in Romania).

But Facebook can take down something I can say in public, meaning I'm not allowed free speech. It's not fair that I suffer all the consequences of a "public space", but none of the rights.