I'm not really arguing from an antitrust perspective. My argument is more focused on the central role that Twitter plays in our political process. If you have a better word to use for that concept than "monopoly", I'll edit my post. But I'm sure you disagree with my thesis either way.
That said, from an antitrust perspective, there are strong arguments against some social media companies, particularly Facebook. Furthermore, there is a barrier to entry for Twitter: the network effect. It's an incredibly strong barrier.
Finally, of course not every big company is a monopoly.
The network effect is a big barrier until it isn't, and this can happen rather quickly.
Digg. Myspace. Snap. Tumblr. Honestly even Facebook itself; how many young people are opening new accounts on Facebook? For now people have moved onto Instagram but this is the only trick I'm aware of pulled off successfully.
You are not entitled to an audience. A monopoly requires an actual barrier, not a social one: i.e. physically limited space in telecommunication conduits in cities, for example.
And I think you've put your finger on it. A lot of people confuse a right to free speech with a right to be heard. They are BIG MAD that they don't have a god-given right to a large platform. Not that they built the platform and not that they contribute to its upkeep. But they still have strong feelings of entitlement.
> My argument is more focused on the central role that Twitter plays in our political process. If you have a better word to use for that concept than "monopoly"
What's the concept then, other than that Twitter is doing things that you don't like? That's obviously not a sufficient argument, and if antitrust law isn't the reason for legal action to be taken against them, what's the reason? The only other legal course I could imagine is significant changes to media laws (which I'm certainly not opposed to, but those seem more difficult to argue for than antitrust action), or perhaps going at it from the other side and changing the laws around how public figures must communicate with the public.
A business doesn’t have to be a monopoly per the current legal definition for it to be subject to regulation. We can simply require social media companies to accept all customers. We can make political views a protected class that prevents denial of service at businesses in general. We can treat these tech companies as utilities because they operate the public town square. There are numerous routes for us to fix the current situation, where a small number of employees controlling these companies with billions of users, can become the sole arbiters of can communicate and what they can communicate. It just requires that we start talking about it, educate people about the problem, and build political will.
We probably can't "simply" do that, in that the Constitution is a document with actual meaning.
I would also note that there are existing platforms that effectively make political views a protected class. It turns out they are not very popular, because most people don't want to hang around with people for whom the 14 Words are an important political view.
In practice, a platform can either have haters or the people they hate. Insisting a platform accept all the haters means you're guaranteeing the people they hate will go elsewhere. That's why Twitter has moved away from the sort of policies you favor. As an example, look at when Milo Yiannopoulos led a wave of racist abuse toward Leslie Jones. Twitter had to choose between keeping her (and a lot of her fans and people who just don't like open racism) or Yiannopoulos and his fans.
So the notion that we should override the Bill of Rights to make Twitter platform everybody is not only anti-freedom, but also won't in practice work. If somebody doesn't like Twitter's rules, they should do what you're doing: post somewhere else. There are platforms ready and waiting to take them.
That said, from an antitrust perspective, there are strong arguments against some social media companies, particularly Facebook. Furthermore, there is a barrier to entry for Twitter: the network effect. It's an incredibly strong barrier.
Finally, of course not every big company is a monopoly.