From the essay "Social Injustice: When Leadership Fails " found here: http://www.marbitz.com/yeshaya.html
What is Social Justice?
Over and over again, as we have now seen, the prophet admonishes his people over their neglect for others - especially others left vulnerable by difficult lives. Clearly, the problem is more than just a relatively isolated weakness in Torah observance (as we might classify a failure to properly observe the laws of, say, kosher food), but stands directly and violently opposed to everything that a Jew is meant to be.
Overwhelmingly, Isaiah's rebukes are delivered in the context of the words "justice" and "righteousness" - משפט וצדק. Before properly understanding the larger subject, we should see if we can't achieve a better understanding of these words themselves.
(This discussion is drawn from "Collected Writings of Rabbi Samson Raphael Hirsch" Vol IV, pp 69-70.) The verb "shafet" (שפט) means to bring something to its proper place. Its noun, "mishpat" (משפט) is hence a process which seeks (to paraphrase Rabbi Hirsch) the satisfactory fulfillment of justified demands. In other words, a human being has a right to his lawful property, status and dignity. When he is illegally deprived of these possessions, he has the right to demand their return. The institution of mishpat - justice - is the tool through which his claim should be addressed.
The noun "tzedek" (צדק), on the other hand, denotes the place where things belong. An act of "tzedaka" - righteousness - therefore, involves acting as one should; disposing of assets as God would have them disposed. Or, in Rabbi Hirsch's own words: "doing one's duty...in accord with the will of God...The fulfillment of the Torah...It takes into account the requirements, rather than merely the legal claims, of the person concerned."
Thus, mishpat acknowledges and promotes the legal rights of individuals when they come in conflict with each other or the state. While Tzedaka places the burden of Divinely-inspired perfection directly on the shoulders of each individual and each instrument of the state, regardless of the purely legal strengths or weaknesses of their opponents' claims.
These principles, tzedek and mishpat, are what we mean by the phrase "social justice." Both of these together - the sense of responsibility towards the rights of our fellow human beings and the sense of obligation to the morality of God's Torah - form the only reasonable foundation for a Jewish community. Their absence is inherently corrupting of both communal and private life.
Now, in light of these tzedek and mishpat ideals, let's take another look at the prophet's particular criticism. Nearly all of the examples from our list above involve abuse of privilege: members of society's powerful classes seeking to enhance their own positions at the expense of the poor and weak. There can be no doubt that crime is wrong no matter who commits it and no matter who is the victim. The poor may no more expect a court to wrongly tilt justice towards them than the rich (see Rashi to Deut. 1:17). But, nevertheless, Isaiah largely ignores the Robin Hoods of the Jewish world. While they, too, may be reprehensible thieves, they are not guilty of quite such a grievous form of oppression. Tzedek and mishpat are primarily the responsibility of the powerful. Of leaders.
Who constituted the powerful classes of Isaiah's world? Kings and government officials, judges, the wealthy, large landowners and Torah scholars (for particular reference to Torah scholars and judges, see TB Shabbos 139a)...anyone whose social standing allowed him some control over others and who could call on highly-placed allies when needed. These are the people who are most tempted by crimes of power and it is to these people that Isaiah's message of social morality is addressed.
