Who decides if Nazis are allowed to put signs with Nazi slogans on your lawn?
Twitter and Facebook are common carriers[0], not private residences.
Edit: 0: By which I mean that are common carriers in practice, not that they are legally recognized as such.
Except they aren't actually common carriers. Some people are of the opinion they should be, but having an opinion doesn't make it so.
I think (based on this) that you mean they aren't legally recognized as common carriers (which isn't what I said, but fair enough; edited to clarify).
Twitter and Facebook are common carriers[0], not private residences.
Edit: 0: By which I mean that are common carriers in practice, not that they are legally recognized as such.