Is there any method to find users who followed someone's account on Facebook?
Unfortunately it seems like you can not check other people's followers. You can only check yours according to https://www.facebook.com/help/201148673283205
But you can check what are the people/pages that a particular person is following, if that person's settings allow you to see it. See https://www.facebook.com/help/401227106619293
So in other words you can check if your friend is following X, if his settings allow you to see it. But you can not get the whole list of all the people who follow X just by going to X's profile.
Use Facebook search
Mark Zuckerberg's followers