I don't know what type of non profit you're talking about but in most cases, the most critical job of non profit board members is to raise money for the cause. I don't know that you'll find people passionate enough to do that on any sites - but if you can it would be sites related to your cause. Better solution would be to find people who are willing to work with you to accomplish your non profit's goals and when you find one with enough time, smarts, connections etc ask them to be a board member.
If you find it hard to find or connect with people passionate about what you're doing, there's learning there too.