> It seems like the only way to really combat this is through closed / semi-closed trusted networks,

This is the only way. The cost of spinning up a new identity on the web (with supporting documents, pictures, etc) is near zero now. There's only 3 or 4 things to really vet people, and those can be faked with more effort.

The only real way to vet someone nowadays is IRL, and that requires non-trivial effort but provides most guarantees you'd want in new participants in an online community.

> I'm getting a sense of being close to that point already and I don't know what the right move is from here to reduce the fracturing of my wider social circles.

I have this feeling too. My guess is that social circles will be less fluid and dynamic. Traditional centers of trust will become more important.