What is a fanlisting?
A fanlisting is simply a list of fans. Fanlistings are created out of love for the purpose of spreading and showcasing fandom. In other words, take this one. This fanlisting is for Mai Valentine. The purpose is to list the people who like Mai. This creates interesting statistics as the members list grows. You'll see how widespread the Mai fandom is, and you'll see how popular she is overall.
You may have noticed, but I'm terrible at explaining things. If I haven't helped your understanding of fanlistings, the TFL for Newbies page should be able to help you a great deal.