I see four options:

1. Server owner assumes all costs of the server. Possible if instance is not huge (ideal on a federated environment) or owner has money and will to do it.
2. Accept donations from users
3. Require paid subscription to access (and thus becoming private otherwise anyone could still access the server from a free instance)
4. Run ads on site. Same as 3, users would migrate.

So I only see 1 and 2 possible

looks amazing!!