Most people who give to charity don't look into what effect it has on the world. They give the money then move on.
Very true. Things like scope insensitivity (
http://lesswrong.com/lw/hw/scope_insensitivity/), where people are willing to pay almost as much to save 2000 birds as 200000 birds, and the fact that people prefer volunteering over donating to charity show this clearly. Volunteering is far less efficient than donating; every hour that you spend volunteering could be ten times as productive in terms of the charity's goal if you had spent it working at a normal job and then donated the proceeds to the charity so some professionals could do your work. "Volunteer vacations" where people are flown to Africa to volunteer are even worse; the net value of one to the charity is likely negative. But people still do it. Why? Because the good that the charity is selling is not satisfying people's utilitarianism; it's socialization. You volunteer because it's fun and because you get to spend time with people who have similar interests to you. This is why arguments that "true charity" must be anonymous are doomed to failure, and charity through donating money is most effective when it's done in group settings like fundraisers, *-a-thons and churches - we are not utilitarians, we are diverse individuals with distinct individual motives.
For something like HUMANLIVES, the problem really is calculation. There are many short-term projects, like food, water and vaccination, that can clearly be shown to save 100 lives or 10000 lives, but what do we do about long term effects? How can we possibly know how many lives creating a school to educate 200 people up to grade 10 level saves? Also, even something as simple as food and water in the long term can get very intractable, and very political at the same time. Some argue that any food and water aid just allows the local corrupt government to steal the same amount of food and water elsewhere, leaving the citizens at just that bare subsistence level that the government needs for its economic base to stay functional. Others argue that it's all useless due to Malthusian considerations. Others, however, argue that making people healthier now also makes people more productive in the long term. It's impossible to weight such concerns while remaining politically neutral; the arguments often descend to the basic disagreements about human nature that political viewpoints are founded on in the first place.