Social commentary is when one acts, or rebels, against a group of people or an individual regarding social issues. In regards to The Glass Castle, Jeanette Wells is speaking out about the issues of child abuse.
Wells' novel is a memoir which speaks to her abusive childhood. Wells and her siblings were starved, lied to, and neglected. Not only that, her parents' immense debt forced them to move from place to place in order to escape their creditors.
Therefore, Wells' memoir is a piece of social commentary based upon the fact that her story speaks out against the abuse of children. Essentially, by telling her story, Wells is able to do a few different things. First, she is able to truly com to terms with her childhood. Second, her story allows others (in similar situations) to realize they are not alone and that hope exists. Lastly, the memoir speaks out against her abusive parents, naming them as abusers.