I know donations are an option, but I don’t think long term donations will be sustainable. Most users will never donate. I don’t think that they are not sustainable. If everything works out to be a properly federated network that is made up out of a lot of small to medium sized instances I think that it would be sustainable. Hosting costs should actually not be too expensive. You don’t end up with millions of users on a single instance causing it to have massive load. And users are generally more willing to contribute financially if they get the feeling of using a platform that reflects their values and is run with their interest in mind.