One of the fastest ways to grow an audience is to work directly with organizations that are already speaking to a large audience. This can take many, many forms, and the more deeply you interrogate your project, the more earnestly you ask questions of your audience and the more interesting and fruitful partnerships you will uncover.
And remember: your audience is not a monolith! They all contain multitudes, and in order to reach the maximum potential of your audience, you may need to work in partnership with many different organizations or design several different strategies.
The brand you partner with will want metrics for success.
(non-profits, foundations, etc) What do you have to offer them?