Why might I see a sudden decrease in my number of followers?
We're in the process of fixing an issue that incorrectly includes inactive or fake accounts in follower/following lists. We want to maintain the best possible experience on Instagram, so we do our best to remove spam, fake accounts and other people and posts that don't follow our Community Guidelines. As we remove these accounts, some people may notice a decrease in their follower/following counts.
Note: This issue was fixed for the majority of Instagrammers in April 2014. By the end of December 2014, it should be fixed for the remaining members of the Instagram community.