The problem was that Google was too afraid to report the number of subscribers to these services, so you couldn't know for sure how many readers of your feed use Google. The personalized homepage has always had a big number of users, but with the latest update of Google Reader, many people switched to Google Reader.
So now Google decided it's time to reveal the mysterious numbers.
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"). (...)
Below is an example of the contents of the "User-Agent:" header:
User-Agent: Feedfetcher-Google; (+http://www.google.com/feedfetcher.html; 4 subscribers; feed-id=1794595805790851116)
If you use FeedBurner for feed stats (this is a good option if you use hosted services like Blogger's Blog*Spot and you don't have access to server logs), there's a good news. Starting tomorrow, you'll be able to see the number of Google subscribers. "This information will show up in tonight's subscriber reports (meaning that most of you will start to see the data on Saturday morning, U.S. Central Time)."