"Mastodon.social runs fine and between individuals and companies it gets plenty of donations"
The donations to Mastodon gGmbH are mostly for the software, not the servers. The donations are supposed to encourage a spread-out decentralised social network, not build up a centralised server.
Also, according to their latest report those donations only pay for one salaried person in the entire organisation (the CEO with €3K a month), with a few other programmers on hourly freelancer gigs.