While the value of ranking sites by their RSS count is the subject of some debate, in the absence of public, uncontested traffic and return visitor data, it is one metric available to nearly all major blogs, especially as FeedBurner has become the online standard for RSS delivery and tracking.
In the last few months, Fedorov wrote me to say, first, that support for branded feeds (i.e. not from feedburner.com) was added, and that feeds which point to subdomain of a blog but are still published with FeedBurner, can be added.
This might sound like a minor change, but this enabled sites like Engadget to be included. Engadget, which didn't figure in the first screen capture, now shows 1.6 million RSS readers, and even minute swings can show adds and drops of thousands per day. From January through today, you can also see TechCrunch increased from 654,000 readers to 782,000, and Mashable from 143,000 to 167,000.
As for me, at the time I clocked in with that post, I had 436 readers, and we're now seeing FeedBurner report 1,028 total.
For fun, I added mine to the site, and you can see my ranking (in the 400s) on the right side of louisgray.com. Does it add a ton of value? Not a lot, especially as you can consider to Rating Burner doesn't have the entire blogosphere indexed. But the database has grown dramatically since it first showed up in January, and it's always fun to see where you sit against your peers and competition, so if you're so inclined, it's real simple to add the button to your template. You can find out how on the RatingBurner Web site.