This is a chicken and the egg problem and it seems like you need developers. But developers might need to see non-profits that inspire them before getting involved.
I agree about the chicken/egg problem. I had considered adding a sort of nonprofit showcase, or case studies to the site, or even just covering them on an associated blog, something. I think one of the first things I'll add next is a "tell us your story" type page where nonprofits can submit info on what they're up to and I can start building up a database of interested organizations and details about them.
I'm attending a couple of big-ish charity events soon and primarily wanted to get the site up and hopefully get a few people signed on board prior to that, work out a solid plan going forward, add what I can in the mean time, and start doing some serious promotion in about a month.
It's a bit of a catch-22 on the promotion end of things, as well. You don't want to promote an empty site, but you need to get users somehow - which is a problem I'm sure everyone on HN who's ever launched a member-based site has face, and I'd love some tips on dealing with it. Right now, I'm mostly pushing it among friends.