How do I use the API to get the number of followers for a given tag?

On Stack Overflow, if you hover your mouse over a Tag a DHTML popup displays the number of followers for that tag.

I have used the Stacks Apps search as well as Google and can't find this piece of data in the APIs.  Am I missing something?  

If "followers" is not a discoverable piece of data, consider this a feature request.  :)