We recently made some changes to Publicize, and unfortunately along the way, messed up some people's follower counts. This number is calculated by adding up the number of people on WordPress.com who are following your blog, in addition to the number of people following you on Facebook, Twitter or Tumblr if you connect those accounts to Publicize. You didn't lose any followers, just the cached number that showed you how many you had.
If your follower count number appears to be incorrect (on the Follow Blog widget, or in your stats), then you can very easily fix it by doing the following:
- Publishing something and Publicizing it out to those services. The number is automatically refreshed each time you Publicize something, so the next time you publish, it should fix itself magically
- If you can't wait, then just disconnect and reconnect each service, which will also force the number to be refreshed.