You might read up on the Trusts era that brought about the Sherman Act[1]. Trains? Horse carriages and wagons were still available, not to mention boats. However, these are not equal modes of travel, though they are "similar" in that they are transportation, and so it's not about internet-based communities per se.

It's the fact that Facebook is "Facebook" that is the problem. That you can't imagine Facebook's equal being viable in any way, other than to mention the wagons and horses of the internet. This is not to comment on a level of sophistication, but features. What can you sell on TikTok through TikTok? It's a category error to equate them with FB. Facebook has cultivated this state of affairs, and they have gone out of their way to interfere with the success of companies it views as competition.[2]

This is even before we get to the use of Facebook to create negative externalities, for which it provides powerful tools. On purpose.

1. https://en.wikipedia.org/wiki/Sherman_Antitrust_Act_of_1890#...

2. https://techcrunch.com/2020/07/29/in-antitrust-hearing-zucke...