Donations or grants would probably be best. I’m sure there are community grants available for doing public good.
I’ve also seen some open source things sell merch, where you buy a hat or something with a particular logo. It’s still donating with extra steps, but it’s a little different
Our instance admin broke it down recently, and it’s actually not too expensive to run. Without wasting money on engagement, growth, data collection, marketing, etc, it’s not actually that expensive to run social media platforms I guess