Could you not think of this as a networking problem. Say you have 10 users with more than 1 million followers and there are 100 million twitter users, what is the probability that some of your other users are following them?
Perhaps for some of the accounts that do not use much credit to search for followers, you could also search for those who it follows. Then on your backend, you can see if this subset exists within a previous list of followers of a big user.
Perhaps for some of the accounts that do not use much credit to search for followers, you could also search for those who it follows. Then on your backend, you can see if this subset exists within a previous list of followers of a big user.