Of course we can. Just look at what happened in a few short years with the spread of Aids in the USA or what Osam Ben Laden was capable of with a small group of followers on a global level. Both have had significant world changing significance.
On a far more positive note, look at the influence of Mother Teresa and she was just one woman. Reflect on the change she has wrought throughout a society of people by the lives she has touched and inspired. Consider the dynamic effect of Ghandi's teachings to the world and he is only one small man in a vast world. Even consider Oprah Winfrey and the tremendous good she has done with her Angel Network and the lives she has touched and allowed her viewers to touch in in kind.
In a population where a video can go viral in a matter of hours such as is the case with the video of the Invisible Children and the villain Koney, in the age of social media such as facebook where we can reach millions in moments, massive change on a large scale is a very real possibility from the efforts of a population of people.
In other words, we can look in the mirror and be the change we want to see in the world. A small pebble creates a large ripple in the pond.