Who pays for the costs of those and why?

There are many Mastodon servers run by ordinary people simply because they want to. And before the shit-show the internet has become, there were many forums and IRC channels, absolutely free, and with 0 ads.

Very low traction on these. Let me know when there’s something that people actually use in tens or hundreds of millions and random people are just providing the infrastructure out of pocket and spending all their time on this without expectation.

Maybe low traction is a good thing. We don’t need social media to be an all consuming addictive mega platform.

Moving the goalposts much? Of course there aren't any free services serving millions currently, how could they, when Facebook/X spends millions to make sure everyone stays on their platform? Which non tech savvy would want to move to a platform without all their friends? That's the gotcha with social networks, once you grow big enough, it is really hard for people to move off of it.

Still, funny how you ignored IRCs/forums that I mentioned. Those were used by MANY people, and could scale infinitely. You are literally arguing against something that has already happened.