I'm fixing it right now, I request users with more than 50,000 friends/followers to please check back tomorrow. The bug would be fixed and you can then safely start using JustUnfollow again.

Here's a technical description of the bug for those who understand a bit of tech : 

I cache friends and followers of a user using memcache provided by the Google App Engine. Memcache has a limit of 1 MB data size that can be cached. I was aware of the limit but never realized this limit would kick in if there are more than 50000 integer ids in my datastructure.

I'm planning to break cache into multiple little caches so that the memcache size limit is honored. Apart from this, the error being thrown by memcache had a severity level of "info" which meant I never caught the error in my logs in production! It was only after a user complained about inconsistent results that I debugged with a large set of friend ids and realized the size limit being hit.

 Update: I have fixed this bug which means it doesn't matter how many followers/non-followers you have. If your account has the API limits to retrieve all your followers/non-followers, JustUnfollow has the power to process your non-followers :)

I would like to thank @LarryWentz for pointing out the above mentioned bug. It's amazing to know that a user with so many followers still cares about those who follow him!