I may be accused of nitpicking here but the Pareto Principle is, although related, distinct from the Pareto family of distributions.
I don't intend to derail the conversation so, rather than typing out an explanation of obscene length, I will refer all interested parties to the following:
Pareto Principle https://betterexplained.com/articles/understanding-the-paret...
Basics of Pareto Distributions https://actuarialmodelingtopics.wordpress.com/2017/05/21/par...
https://subversion.american.edu/aisaac/notes/pareto-distribu...
Example of some limitations in the application of Pareto Distributions https://arxiv.org/pdf/physics/0504217.pdf
"80% of the work is done by 20% of the workers".
https://en.wikipedia.org/wiki/Pareto_principle
Where this comes into play with regards to your thought experiment... is how the larger the playing field, the fewer the large players are - they tend to get "concentrated".
Plenty of Facebook alternatives exist... but why go to them? No one uses them. Large chunks of friends/family/acquaintances aren't "there" - they are on Facebook.
Plenty of authors other than Steven King exist... but given a few spaces on shelves at kiosks, who's going to get those spots? You can guarantee King will get one of those few spots...
The majority of activity is going to condense down until a few spots. Why? Because that is where everyone else is. That's where the skilled "creators" are.
The question becomes... how do you make sure those spots are fair, open, balanced, secure, etc.
Data in the hands of a few companies? What happens when they all lean left - politically? What happens when one of them gets hacked? When a "foreign actor" learns how to game that system?
I've thought for the last decade that things will continue to get "worse" before they get "better". How much worse? How can we make it better?
Million dollar questions.