Space usage under flatpak is highly overstated. It only takes a noticeable amount of storage if you only use a couple of flatpaks, cause all the dependencies are used for a single package, once you start using flatpaks as the main mean of installing “applications”, the space required start to decrease because the dependencies are shared between multiple apps
Even if a social network loses 99,99% of the user base due to charging to use it, those left are the ones that see no problem paying to use it, so they are more likely to eat up some insane pricing, which would help recoup losses from a smaller user base. Basically whales.
I think the only way to try to kill a social network is by going full scorched earth on it. Remove all your comments, or change them to be an annoying copy pasted comment about why you’re getting off the platform. And even then I don’t think it is helpful, I did that with Reddit but was forced to leave technical posts intact because I feared I might prevent someone from solving their issue.