The bystander effect/Genovese syndrome: http://en.wikipedia.org/wiki/Bystander_effect

Perhaps the effect is quite different when there is just one other bystander, as opposed to a whole crowd of them.

Yes, but there's a critical difference. In this case there's a clear recipient - someone asked to do something/reply. In the case of the Genovese effect, no-one is specifically asked to do anything, and no-one does.

Correct: http://en.wikipedia.org/wiki/Diffusion_of_responsibility

You mean like this: http://www2.selu.edu/Academics/Faculty/scraig/gansberg.html

But this technically isn't diffusion. It's concentration of responsibility.