I wouldn't be aware of this Earth Hour US if I am not using Google. Earth Hour explained why Google home page had black background. The initiative was kicked off in Sydney and spread around to different cities. This would be small thing we can do by switching off our lights for an hour from 8 - 9 pm.
The video in below link shows how much energy saved during this hour. First time, something was so easy to get a great satisfaction. Just switch off the lights...nothing much..