I would think that one of the most powerful impacts Jacob Riis had was with the idea of depicting "how the other half lives." In an age where industrialization, economic wealth, and the desire to gain material progress dominated the American social discourse, Riis did not hesitate in exposing the other side to this pursuit. The cramped conditions of the cities, the horrific conditions of childhood, and the pain and suffering such progress cost in terms of human life and living were all subjects of his photographs. I think that the impact Riis made was to raise awareness of this condition and prove the counterpoint to material progress in America at the time. In terms of legislative change, perhaps his impact is not as widely felt, but as a voice of the Progressivist movement in America, Riis ability to ensure that more voices are included in the social dialogue is highly relevant.
Jacob Riis was very concerned with the treatment that immigrants received when they came to America. He did a lot of research and reporting of the conditions that immigrants faced such as work issues and living conditions. He wanted to improve the living conditions that these individuals had and address working issues.
He was a very popular writer and even had the esteem of Theodore Roosevelt who offered him jobs that he refused. He refused so that he could continue his work as an ambassador to the poor. He did this by reporting and writing books to keep people informed.
Jacob Riis was himself an immigrant and faced some of these tough issues. This is why he was so passionate about his cause.
All of this is why he had a social impact on America. He educated people on issues that immigrants faced when they came to this country.
It is hard to say that Riis in particular had any social impact. However, Riis and others like him certainly had an impact on their time.
Riis was one of the muckrakers, the people who exposed poor conditions in various aspects of American life. In Riis's case, he made the public and powerful people more aware of the harsh conditions in which poor people in the cities lived. The attention that this brought helped to cause the Progressives to reform the way things were done in American cities. The Progressives pushed for better, more honest city governments and better laws to ensure that housing was safe, clean, etc.
So in that way, you can say that Riis had an impact.