Case in point, Lebron James. According to analysis by Big Lead Sports, only a third of King James' followers appear to be real active users. Serena Williams did even worse, at less than a quarter of her users. She appears to have more 'fake' followers, than real ones.
That's not to say that they necessarily paid for those users to follow them in bulk, but it does indicate that the engagement of their followers may be lower than others.
On the other side of the coin, celebrities like Ashton Kutcher and Shaq, where were among the first to join Twitter, naturally have more followers, real and fake, because they have been their since the beginning.
But it does show the potential value of a lesser-known athlete or blog or individual who's audience is really engaged compared to a big-name property that drive a lot of low-quality eyeballs.