In my opinion, the invention of the printing press (with movable type) was one of the most important ever. It allowed information to become available to many more people. This helped cause a great deal of social change.
Before the printing press, it was very difficult for ideas to be spread. A person with an idea would have to speak it to someone else. Or they would have to hand write it and then have it copied by other people working by hand. In both cases, transmission of ideas was slow and the ideas got corrupted by being copied over and over.
With the printing press, new ideas (like Luther's ideas that started the Reformation) could be passed around quickly and easily. At the same time, information (books) became much cheaper and could be afforded by a much greater section of the populace. Both developments led to a population that could have access to many more ideas -- this helped allow for social change.
The printing industry plays a vital role in bringing social change since it spreads and delivers messages. Printed materials, unlike the online articles or TV programs, can reach all places and are available anywhere. Printed materials can reach far flung rural areas where there is no available electricity or internet connection. For example, consider non-profit organizations. They usually resort to custom sticker printing services or poster printing to make the public aware of their advocacies.