See Who Un-Followed You on Tumblrby Andrea Ruiz
Tumblr users who follow your blog syndicate all of your blog posts on their dashboards in chronological order, with your most recent posts listed first. The site keeps a real-time list of users who follow each of your blogs in the blog's dashboard. As of January 2014, Tumblr has no native feature that automatically notifies you when another user unfollows your blog, so you must keep track of which users unfollow you manually or with a third-party tracking service.
Viewing Your Followers List
You can view a list of your followers by logging in to your Tumblr account and selecting the blog whose follower list you want to view from the drop-down menu. Then, click the "Followers" tab to see a paginated list of your followers. Tumblr lists your followers in chronological order, with the most recent account to follow you listed first. Depending on the size of your followers list and how well you remember your followers' usernames, you may be able to keep track of who unfollows you by checking the number of followers you have and going through the list to see who's missing when the number reduces.
Tracking Individual Followers
You can quickly check to see if an individual user still follows you by entering the username in the search field on your followers page and pressing the "Enter" key. Tumblr displays a pop-up notification below the search field letting you know whether the user follows your blog.
Keeping Track of Followers
If you have a followers list that's too large for you to keep track of at-a-glance, or if you'd like to keep track of your followers list in hard copy form, you can create a spreadsheet in your preferred spreadsheet program for this purpose. You can copy and paste your current followers list in one column and save the spreadsheet. When you notice that a user unfollows you on Tumblr, you can copy and paste your new followers list in a column next to the original list in the spreadsheet and compare which usernames are missing. This method requires regular upkeep to ensure that you have a current copy of your followers list against which you can compare any changes.
Using Friend or Follow
FriendorFollow.com is a website that enables you to track which users you follow that don't follow you back. First, create a free account with an email address, and then authorize the service to check your Tumblr followers list. The site displays an analysis of which users you follow but don't follow you back, as well as followers you don't follow back. This is a particularly useful method for tracking which users unfollowed you if you have the regular policy of following back everyone who has followed you. If so, any users who don't follow you back are users who unfollowed you some time after you followed them.
As of January 2014, there are no native automated means to quickly view your entire list of followers on one page. If you want only a snapshot of your follower count and aren't interested in tracking the activity of individual followers, you can quickly view of the number of followers your blog has by glancing at the number in the Followers tab on your blog's dashboard. This number is the number of users that follow that blog.