Jacob Riis’ book “How the other half lives” documented the living conditions in the New York slums in the late 19th century. He was a journalist and self taught photographer, Riis used magnesium flash to photograph the interiors of the slums which would otherwise have remained invisible. He only photographed from 1888 – 1898 and stopped photographing when he had enough material to illustrate his books and articles. His contribution to the movement of social reform in the US is thought to have started Social Documentary Photography genre. MOMA has a collection of his work.
Lewis Hine was another socially aware photographer who documented child labour in the US between 1908-12. His photographs with original captions can be found here:
Both of these men were committed reformers and their influence can certainly still be seen today in the work of many documentary photographers. A modern example of this type of campaigning documentary can be found in on-line magazines such as Life Force