According to Facebook, "You can become eligible to turn on a top fan badge by being one of the most active people on a Page, which can include watching the Page's videos, liking or reacting to its content, and commenting on or sharing its posts."
There are no specific criteria for the awarding of a TopFan Badge. I imagine it could be total engaging activities or a weighted combination of them.
Badges might be limited to a certain number, or a defined percentage of the Page population. Or they could be awarded once you meet a specific threshold, for example, x number of comments per week.
The TopFan Badge is recalculated each week and can be "lost" and regained from week to week.
We use the TopFan Badge to assign weekly privileges or priorities for some of our activities. For example, the weekly Q&A segment, [email protected], gives priority to questions which come from TopFans. TopFans often get advanced notice of special giveaways or announcements. We use the TopFan Badge to honor those who are active participants.