Nominally I would say that Twitter and Facebook are private property and in the United States your freedom of expression only pertains to public property. However, the complexity comes because the government both enables the idea of lawsuits against Twitter and Facebook and also bars them.
For the moment though, I would say my usual right wing admonition applies: If you don't like what Twitter and Facebook companies do, either find another company or start your own. It would be helpful if conservatives remembered that such a rule applies to them too. In general, it would be helpful if they remembered that shockingly everyone has a right to vote, that the Constitution applies to everyone, and they are not the only Americans and Patriots despite their own media telling them that.
It would be even more helpful if the left wing realized that its own speech is just as insightful. After all, the wave of violence and rioting protests that took place over the summer certainly was fueled by a steady diet of academic coursework, political punditry and media coursework designed to sell the idea that the entire country is racist and therefor everything is subject to destruction.
The question really is, if the flag is a national symbol and things like Best Buy and Rite Aid are legitimate targets for destruction in the name of protest and social justice, then why not the nation's capitol? The nuance is that, among Trump supporters, even though the mythology of this election is a fraud is laughably incorrect, there is also truth among that crowd of generations of abuse abetted by the government and on a number of policies, particularly linked to wars and resource extraction and abandonment. This is not dissimilar from blacks being lead to riot because everything wrong in their lives is because of white people, of whom a large portion either don't care, or, are actually working for immigrants and global multinational companies where race is essentially not a distinguishing marker of success.
The truth of the matter is, pushing racial and cultural buttons is profitable for left wing and right wing media alike, and the riots of the summer and the riots of this election are caused by the exact same type of person.