I hate the term "Protest Singer". Woodie Guthrie never called himself a Protest Singer. Nor did most of the people mentioned in the thread. It's just a marketing label that tries to fit people into little boxes.
Everyone seems very geared to the American scene so far - if we'rer talking about where the most powerful songs about injustice came from, Ewan McColl and Christy Moore should be in there.
But I thionk Amos was right: "The question is a troll, beyond question." Sticking people in competitive lists like this is a bit of a waste of time.