Google has announced in the Official Google Reader blog that they will be publishing feed subscriber numbers in the crawler. Publishers have been wanting this information for a while.
Publishers have been asking us to report the number of users that are subscribed to their feeds in Google Reader. This is something we've been wanting to do for a while, but with all the products that use feeds at Google, corralling the data in one place was like herding cats. So herd we did, and as of today, our crawler reports the number of Google users subscribed to the feed. The count includes subscribers from Google Reader and the Google Personalized Homepage, and in the future may include other Google products that support feeds.
Feed publishers can find the number in their server logs. Google has posted an explanation about how to find subscriber counts here in their FAQ.
Does Google Reader report subscriber counts?
Yes, Google Reader reports subscriber counts when we crawl feeds (within the "User-Agent:" header in HTTP). Currently, these counts include users of both Google Reader and the Google Personalized Homepage, and over time will include subscriptions from other Google properties.
The "User-Agent:" header of our crawler includes the name of our crawler ("FeedFetcher-Google") along with its associated URL, the subscriber count, and a unique 64-bit feed identifier ("feed-id"). You might see multiple requests for the same feed with distinct "feed-id" values. This happens if the same feed is referenced through multiple URLs (for example, because of redirects). In that case, you will need to sum up the subscribers to a feed that have distinct "feed-id" values to determine the total number of Google subscribers to the feed.
Below is an example of the contents of the "User-Agent:" header:
Publishers may also be interested to know that MyYahoo recently started reporting subscriber counts again -- after a delay of several months. If you are not using Feedburner you might be interesting in this page on Yahoo's publisher.yahoo.com site which tells you how to track MyYahoo subscribers in your server logs. Most other news readers like Bloglines, Newsgator and Netvibes also provide subscriber counts in server logs.