I think it's a mistake to view "donating" as
  meaningful response to many current social problems.

It probably depends on what problem you hope to solve, and what the other activities you're considering are.

A guy on minimum wage can ladle out soup at a soup kitchen every bit as well as I can.

But can he convince my elected representatives that I care about soup kitchens? Or make soup kitchen users into my personal friends? Not so much.