Anyone who has run a donation based service will tell you whatever % of users you think contributes back divide that by 1,000 or 10,000 to get the actual number.
The elephant in the room here is how you ethically get people to onboard without an existing community / fomo / money.
The trick is getting the content creators there, but most of them are ultimately and fairly interested in making money, and your new platform wont have that for them.
Bluesky has done alright, but that was a black swan event Elon Musk inspired.
It doesn't have to though, we could train AIs that push back or even coordinate with a human therapist similar to how self checkout lines still have an attendant.
Yeah one key missing feature is subreddit creation and moderation.
Without all the free labor from moderators curating content for topics with different tastes and ideas about what the community should be, it's going to be a struggle to build a community.
Even if you go on like Reddit if you post certain things in certain subs you'll get invited to private subs talking about whatever niche topic.
Same with Discord, lot of small communities there.