That's an excellent question. I worked for INN (The Institute for Nonprofit News) for a few years. At the time we had ~140 nonprofit news organizations as members and the core of our work was helping them get to a place of financial sustainability.
Typically, success looked like:
* recurring revenue from memberships
* events
and sometimes
* partnerships or even acquisition by a public media organization
(it's their call of course, I'm just an observer)