Reach one person, reach thousands more
Not all your audiences will be on Twitter.
BUT there may be audiences out there who previously you've been unable to reach. They may be different audiences to those who respond to other channels - newsletters, Facebook, direct mail or other.
If you can reach opinion leaders and opinion informers, that's' major. This enables others to help your NFP to carry out its mission.
Reach one person, reach thousands more... ultimately the "Go viral" effect.