Except maybe in totalitarian states, you cannot have a single publisher. Most large cities had multiple independent newspapers.
In recent years, we saw a surge of concentration in newspaper and television ownership. However, this was accompanied by a surge of online journalism. The total number of publishers increased, if nothing else.
You can more easily have a single carrier/distributor than a monopolistic publisher. For example, the same delivery service provides me my newspaper as well as a range of competing newspapers. The delivery man does not much care for the content of my newspaper. A few concentrated Internet providers support diverse competing services.
The current giants (Facebook, Twitter and Google) were built initially as neutral distributors. Google was meant to give you access to all of the web’s information. If the search engine is neutral, there is no reason to have more than one. If twitter welcomes everyone, then there is no reason to have competing services. Newspapers have fact-checking services, but newspaper delivery services do not.
Of course, countries like Russia and China often had competing services, but most of the rest of the world fell back on American-based large corporations for their web infrastructure. Even the Talibans use Twitter.
It has now become clear that Google search results are geared toward favouring some of their own services. Today, we find much demand for services like Facebook and Twitter to more closely vet their content. Effectively, they are becoming publishers. They are no longer neutral. It is undeniable that they now see their roles as arbitrer of content. They have fact-checking services and they censor individuals.
If my mental model is correct, then we will see the emergence of strong competitors. I do not predict the immediate downfall of Facebook and Twitter. However, much of their high valuation was due to them being considered neutral carriers. The difference in value between a monopoly and a normal player can be significant. People who know more about online marketing than I do also tell me that online advertisement might be overrated. And advertisement on a platform that is no longer universal is less valuable: the pie is shared. Furthermore, I would predict that startups that were dead on arrival ten years ago might be appealing businesses today. Thus, at the margin, it makes it more appealing for a young person to go work for a small web startup.
I should stress that this is merely a model. I do not claim to be right. I am also not providing investment or job advice.