I've been helping change the world on facebook (along with getting back in touch with people from the past; old friends, schoolmates, crushes and even an old babysitter).
On facebook you can promote specific charities by joining groups and have pages (Kiva has a page!), and there are applications that allow you to 'click' for non-profits. For example, The Hunger Site has an app. you can put on your facebook page. You can help save the rainforest with 'Lil Green Patch'.