Beyond the concerns of social equality in terms of gender and race, there is a concern in Of Mice and Men about economic equality, disenfranchisement, and economic exploitation.
This is, perhaps, the most obvious social concern presented in the book, and it is fit like a puzzle piece into the jigsaw of a broken/disfunctional social code that silences and alienates those who lack economic power.
We also have the social concern of ustice and moral responsibility in the examination of the two deaths in the novel. Curley's wife is killed at the hands of Lennie - we might rule this manslaughter, or perhaps Lennie would be able to take an insanity plea. George shoots his friend with malice aforethought, making his act murder - or would we wish to appeal for a mercy killing? The judgements are left to the reader to consider their own view of justice.
I believe that some of the social concerns for the novel are (as mentioned in previous postings) sexism and racism. Although, one could look at social themes in another way as well though. The social theme of private sphere (social sphere and private sphere roles) roles is in question. George is basically the caregiver for Lennie- a role typically allotted to women. Secondly, Curley's wife maintains a role that is outside of that of a typical private sphere- she does not seem to be one that maintains the house or rears the children (not that there are any children and she does not carry herself as one to do housework). She, instead, seems to be more interested in the public sphere- one typically suited for men at this period of time.
We learn also the futility of the American Dream to the real population of the US at the time when the concept was first framed. There is no success or progress for any of the characters in the novel; each individual has barriers to their ideal future that no amount of dreaming can overcome.
Steinbeck's belief in the fraternity of men is prevalent throughout Of Mice and Men. It is through this solidarity, then, rather than isolation, in which the men can raise themselves. For instance, because the workers were migrants, they never could form any kind or organization or union, thus giving themselves such leverage against the companies for whom they work.
The first post describes the most important social concern in this book. However, the book does also touch on issues of racism and, to some extent, sexism. The characters of Crooks and, to some extent, Curley's wife have been stunted by the way in which society pushes them into certain roles. Steinbeck shows a great deal of concern for these "minorities" at a time when such concern was not really all that common.
Steinbeck is aware of the social conditions that plagued migrant workers during the 1930s. He recognizes that individuals are enduring tight and challenging economic conditions. However, he is also aware that individuals do not have to sacrifice solidarity and collectivity despite material challenges. The "bindle stiffs" that are shown in the novel are representative of the migrant worker population, in general. Steinbeck is concerned that the transient and isolating nature of such work precludes the idea of working together and remaining as a collective unit. One of the reasons why the dream of Lennie and George is so appealing to Candy, and even Crooks to an extent, is because it is so different than the reality of Curley and Carlson, men who move from one job to another and care for little anything other than own condition. In the end, the social concern for solidarity, something that Steinbeck reveres, is something that is expressed powerfully throughout the novel. When George has to sacrifice Lennie and Carlson wonders, "What's gotten into them?" is an ending that reflects this concern.