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There is not a single Mastodon &server out there that have increased donations or reach a sustainable level after they reach a few thousand users.
Also, there are not enough admins around “doing it because they want to” if we want the Fediverse to grow a few millions users.
Instagram has 2 billion users, Pixelfed largest instance has less than 200k active users. We would have to get 10 THOUSAND admins in order to compete with Instagram.
Newsie.social has (had) 20k active users, mostly professional journalists. It has been threatening to shut down due to lack of funding for two years already. Every month their admin needs to beg around for people to donate.
Fosstodon started with enough donations that they could even send some of their money to upstream projects. Nowadays they are invite-only because they don’t get enough funding to sustain infinite growth.
Moth.social was active while they were sponsored by Mozilla, they are shutting down in March 12th due to lack of funding.
I could go on.
This is just wishful thinking. Go ahead and open an instance with open registration, see how long it will take for you to regret it.
And there is a huge number of admins that got users and then burned out due to harassment, spam, entitled users asking for/against federation due to petty drama…