If it was fake, and this many people were participating, chances are someone would have found out by now, andthe company that did it would be so up to its ass in fraud charges that it wouldn't be worth the risk. And, several of the click to donate sites are organizations that existed long before the clicking. Care2, for example, is a large and well established non-profit that does a "click to ___" sort of thing.
Oh, and if you can't figure out the math... even if you only donate a few cents per day, it ammounts to a decent amount of money when tens of thousands of people do it.