It is necessary at all times to distinguish whether we are talking about humans or rational agents, I think.

<humans> If you expect that larger organizations mount more effective marketing campaigns and do not attend to their own diminishing marginal utility and that most people don't attend to the diminishing marginal utility either, you should look for maximum philanthropic return among smaller organizations doing very important, almost entirely neglected things that they have trouble marketing, but not necessarily split your donation up among those smaller organizations, except insofar as, being a human, you can donate more total money if you split up your donations to get more glow. </humans>

<rational agents> Marketing campaign? What's a marketing campaign? </rational agents>