It doesn't measure whether the tweet is positive v negative, which surely has a large impact.<p>I think the 'reach' figure is reaching as well, because I am guessing they just counted the number of followers the person who tweeted has, but of course, there is no way to know that every follower read that tweet.
And once again the take-away message from this: Sex and fuzzy animals sell (see the H&M/David Beckham in his underwear and Coca-Cola Polar Bears and the tops spots).