Comment by BrenBarn
Those are important reasons, but there are other reasons as well, such as concentration of market power in a few companies, which allows those companies to erect barriers to entry and shape law in ways that benefit themselves, as well as simply creating network effects that make it hard for new social-web projects to establish a foothold.
That's an even harder problem to solve. I do agree we should make sure that policy isn't manipulated by vested powers and make things even harder to compete with.
But network effects seems to be a natural phenomenon of people wanting to establish a familiar routine. I look at Steam as an example here, where while it has its own shady schemes behind the scenes (which I hope are addressed), it otherwise doesn't engage in the same dark patterns as other monopolies. But it still creates a strong network effect nonetheless.
I think the main solace here is that you don't need to be dominant to create a good community. You need to focus instead on getting above a certain critical mass, where you keep a healthy stream of posting and participation that can sustain itself. Social media should ultimately be about establishing a space for a community to flourish, and small communities are just as valid.