One of the metrics (albeit a somewhat weak one) of a given tag's "health" is how many followers it has. I believe this is the count of how many users have the tag on their "Favorite Tags" list. You can view this information by hovering over a tag in the tags list.
However, I can't seem to find a way to get this information via the API. Is there some way that I'm not seeing, or is this absent? If it is absent, is it possible to have this feature supported by the API?