You're effectively saying that Google failed, therefore nobody can succeed. This suggests that Google threw all its might and resources behind Google+ and still came up short. I don't think that's true, but even if they did, throwing sufficient resources at the problem was not the issue, the problem seemed to be poorly understood within Google. They seemed to be building "Facebook but Google-branded, but also somehow not Facebook" and then when it was struggling early on instead of trying to fix it, they just went "Ok all Google users have a Google+ now" and acted like they were blowing up.

There is a very interesting counterexample. A company that Facebook saw as enough of a threat that they bit the bullet and spent what was at the time an eyewatering amount of money on a company with a product that was built by a tiny team - Instagram.

We can't ever know what would have become of Instagram had it not been acquired, maybe it would just be the aspirational selfies-and-travel-pics app or maybe it would grow and become something altogether different. But it is certainly clear to me that the failure of Google+ does not mean someone can't build a company that could grow to rival Facebook. They may seem dominant in social media now, but companies which have been completely dominant in their field have been known to totally collapse - remember Nokia?

Absolutely there is a problem. That was my point. If we fix the monopolization of these industries, we solve the moderation issue as well via the free market.

Obviously solving the kind of monopolies created by social networks is hard. The best proposals I've heard is forcing them to open up their social graphs/APIs to competitors, but that's not without its own issues (e.g. bad actors siphoning off user data, like Cambridge Analytica).