"Social activists are people who act as the conscience and voice of many individuals with in a society. They address and challenge their nation on ethical and moral issues of human rights, animal rights and environmental issues" - wikipedia, Wikipedia:Social activism, redirected to Activism. My rule is that if WP wants to ban it or hide it, it is almost always worth something.
I would definitely make the distinction from the author of the above, who then went on to list a grand and very fine group of world-shakers, that Social activists are the grassroots people, especially those who just do it without necessarily asking others to. This is the style of the Back-to-Nature movement, and many other Hippie endeavors