I think it depend. If you are not good at SEO like me and you count on social networks for traffic, then you have to know what people are looking for in your social network. For example in Pinterest 80% of the users are female mid age. So weight loss/healthcare niche is perfect. If you have tumblr for example, most of the users are horny teenagers - porn will be perfect. But from my experience " fight cancer " niche pay the most. I just don't feel like making money from that niche... people that will read those articles, they are in pain, they have someone closer to them, who have this f*cking disease, they will click on your advertisment, because they are looking for the answer. If you decide to pick weight loss/healthcare and you need help with Pinterest, PM me i'll invite you to my boards and give you some tips how to get 2000-4000 k UV daily with little efforts. Sorry for my english. Good luck !