It depends what you mean by change the world. In the world of popular music for example then yes, small groups of people do change the world. Looking back to the 50s when rockers like Jerry lee lewis, Little Richard and others came along they certainly changed the music world forever.
Likewise The Beatles. And Bob Dylan and one or two more.They helped change a generation. They're not acivists in the pure sense but they sparked change in society and culture for sure.
The hippy generation gave rise to the Green/Eco movement and without that,arguably, we wouldn't have had any attempts to be sustainable or to treat Nature with respect. Greenpeace has a few core pioneers who help bring issues that would otherwise be swamped in corporate-speak to the world's media.
In the UK a couple of hundred years ago a small group of union workers formed the Labour Party and they helped change the way working class people were treated in the early industrial times.
There are other examples of small groups starting off with an idea and gradually getting their message out to the masses. Think about the early Communists/Marxists in Russia. They certainly changed things!! Communism didn't last long but it did have many adherents and was considered a viable alternative to capitalism.
Nowadays I think the internet and social media gives activists the chance to change things and stir up political action. The Wikileaks founder could be considered an activist - changing the way we see the darker side of governments in action - we need these kinds of people to help bring issues and problems to the surface so that we can all see what's going on and make up our minds.