The 1700s and 1800s version of Twitter was standing in the street square, handing out pamphlets, and screaming your message. You were protected to say what you wanted to say via free speech. The 2018 version of that is online through tools like Twitter. By refusing to acknowledge this, we're actually experiencing a dramatic practical reduction in free speech rights without ever technically violating the Constitution.
It's really the ultimate loophole to the ultimate problem. Want to limit free speech? Simply remold society so that communication patterns across the entire nation change and become controlled by private corporations. Done.
A particularly dystopian and extreme imagining of this would involve the CEO of a company like Facebook running for President and controlling speech and news to an unprecedented extent in favor of his/her candidacy.