I'm ashamed to say my Mum brought my attention to Thunderclap. God only knows how she found out about it. Essentially it's a way to get massive social reach. You think of something, a cause for example, you wanted tweeted, and then (hopefully) people will sign up to your cause, and on a give date at a given time, the tweet will be automatically tweeted by all who signed up.
Or, as they say, "Social media is an easy way to say something, but it’s a difficult way to be heard. Thunderclap is the first-ever crowdspeaking platform that helps users be heard by saying something together. It allows a single message to be mass-shared, flash mob-style, that rises above the noise of your social networks. By boosting the signal at the same time, Thunderclap helps a single person create action and change like never before."