Loads of organizations have been running core infrastructure from donations for decades now. If you don't need to write new code, and don't have a pressure to actually turn a profit, and have people with enough investment in the community to donate time doing moderation / sysadmin tasks, then it should be easy to get enough donations to keep a server up and running.

In any case, one of the core functionalities of most federated software is the ability to migrate your profile somewhere else.