But I don't want nets, I want people to be helped by nets and that requires a lot of work. So the efficiency of a charity is a way of saying that if you give $10, that means $10*efficiency nets in hands of people that need them.

Although I partially agree with you, it is easy to show why your reasoning doesn't work. Let's say I create a charity to provide nets. I pay myself 99.99% of all donations and use the rest to fly somewhere myself and hand over 10 nets so I get a free vacation as well. And this is not a wholly theoretical discussion, I am sure there has been a charity like this.