The trouble with paid social media is that the value is in the network, and becomes much less attractive if lots (maybe 90% of users) don't pay, and hence are removed from the service.
You could do freemium, but you'd make a lot less money (FB would anyway, maybe this would work for Twitter) without reducing your support costs.
So yeah, I'm not sure this would work in the current setup (even global privacy laws with teeth will move from individual level ads to cohort level ads). The trouble is not that subscription services are worse, it's that ads are super profitable if you're a really big service.