According to the Facebook statistics page [0], there are over 400 million users on the site. The same page states that about 70% of those users are from outside the US. That suggests that 30% of Facebook users (about 120 million) reside in the US. Let's assume that the entirety of her fan base is within the US. That means her fans only represent (at the _most_) approximately 5.59% of US Facebook users. I wish I had data per state so I could pin this down more thoroughly (wrt to ratios of Megan Fox fans to state populations), but going on just what I have seen so far I am admittedly suspicious of the gentleman's data.
You're right to be suspicious, there is a bias towards the more popular pages, since they show up more frequently on people's crawlable public profile pages. Only 20 liked pages are shown for each person, and FB apparently pick the most popular.
