1/ "First Amendment" is trending because people are morons. While the terms "free speech" and "First Amendment" are often used interchangeably, they are not exactly the same. You can infringe free speech even when government is not involved. 
2/ For example, when terrorists murdered cartoonists, we called this a "free speech" issue, as terrorists tried to suppress speech with violence.
3/ Free speech is based on the idea of tolerance of people you disagree with, that you give them equal opportunity to express their views no matter how much you disagree with them.
4/ When Facebook and Twitter censor content, it's therefore a free speech issue, even it's not a First Amendment issue, since they aren't part of the government.
5/ However, in today's incident, Twitter isn't censoring anything. They are simply adding a label to Trump's unsubstantiated claims saying that they are "unsubstantiated". This in no way suppresses Trump's ability to speak freely.
6/ Ironically, though, it is indeed a First Amendment issue, because Trump is threatening to use his government powers to infringe on Twitter's freedoms.
7/ So "First Amendment" should be trending, but for the opposite reason that morons are using it.
