I’m not sure that you’re not underestimating the cost for these sorts of services. The only long running sort of social media (BBS) I know of that is and has been for decades straight pay for access is The WELL. And they need to charge $15 a month.
Of course, they’re not anywhere near the scale of Facebook, but they are similar to a mid sized fediverse server from what I can tell. I honestly think the actual thing going on is most people find value in a free service, but don’t find enough value to pay what it’d cost to make it a straightforward pay for service business.