I checked with http://www.givingwhatwecan.org/about-us/our-members.php

Of 64 members, 18 listed themselves as philosophers - or budding philosophers.

I think this further supports the "signalling" theory of charitable giving. For one thing, pledge makers are listed publicly. Those individuals are among those most likely to benefit through having a reputation for being a goody-two-shoes. The pledge appears to be totally non-binding. That would appeal to those wanting to signal generosity - rather than actually wanting to commit themselves to it.