1) My family is on Facebook. I am not. ;-) It's already the case that Facebook, Instagram, and Twitter for example have different "crowds", though there's some overlap.
2) Somehow humanity survived without these networks pre-2007ish.
3) I have no problem with the existence of many smaller special-interest social networks. Those are manageable, and we had them before the rise of the BigCos. The problem is with the existence of a few all-encompassing, world-consuming, general-purpose social networks. They have proven to be completely unmanageable and toxic.
"Abolish Gmail", for instance, doesn't mean abolish email providers. Not at all. And it doesn't mean that "the next gmail" will replace it. Healthy competition is good, unhealthy monopolies are bad.
What is my "monopoly theory"? I clarified what I meant: "A platform so gigantic that it's impossible for humans to intelligently manage it". We don't really have a good term for a corporation that has undue, oversized market power. Pedants like to argue "so-and-so is not literally a monopoly!" but this is just wordplay and doesn't really address the serious social issues involved.
> Your point 2 says you really want to join the Amish.
Maybe!
> You mean smaller platforms like Parler or 4chan?
I mean smaller platforms. It would be ridiculous to suggest that your cherrypicked examples are representative. You can make anything look bad by cherrypicking.