It's always hard when you realize the largest companies, the ones with products that you just can't seem to escape, are also Giant Corporations Who Care Only About Profit. Or at least that they have practices that, shall we say, aren't-quite-nice.
M & M's, owned by Mars, Inc, is the largest chocolate and candy company in the world. Alas, they do not buy fair trade chocolate. And that's sad, because the cocoa industry is plagued not only by poverty, but by child labor practices.
So the biggest bang for your fair-trade chocolate buck is to demand that M & M's support fair trade. Boycott M & M's, made with chocolate picked by impoverished African children! And in any case, that cheap milk chocolate tastes awful once you get used to really good dark chocolate.