I think sponsorship is a great idea. Look at the success Meetup has had with this strategy. The nice thing about sponsorship money is that once a contract is signed, the revenue will be consistent month to month.
The payoff for the sponsoring company will be the good reputation it gets from that developer ecosystem. There could even be a way to connect this sponsorship to recruiting developers from that ecosystem as employees (the bonus being that they are already familiar with this project)
I made a ticket for this:
What about a sponsorship component? Facilitating companies sponsoring a programmer? Their github page gets a logo or something, and they get a bit check each month. It's not a new model. Skateboarding, for example, which has no practical use aside from entertainment, often works this way I think. Many skaters earn their living from sponsorship and make next to nothing from competitions or whatever else.
If there were a major github competitor (maybe there is, I'm not so much in a position to know), the rivals might also pay these figures to work on the projects they control on their platforms. It helps Github immensely to have all these projects on its platform.