A few thoughts I had while perusing eval.me:
1. There doesn't seem to be information about what charities you partnered with on the anonymous site pages. I suppose this could be contractual, but it was one of the first questions I had.
2. I was confused by the use of "Sign Up" and "Sign In" together; at first glance I thought there was an issue with your site and there were two Sign Up buttons. Maybe "Sign Up" and "Login" or "Register" and "Sign In"?
3. I haven't investigated thoroughly, but wouldn't a charitable incentive inherently skew your survey pool? Although upon consideration, a product-based incentive may do the same thing - just in a different direction.