Search Engine Land posted an interesting piece that analyzes who the various aggregators count RSS feed subscribers.
Bloglines will count anyone who is currently subscribed to your blog even if they have not viewed your blog feed ever as a subscriber.
Google will count anyone who is currently subscribed to your blog even if they have not viewed your blog feed ever as a subscriber.
Microsofts Live.com does not appear to share subscriber numbers.
Netvibes will count anyone who is currently subscribed to your blog even if they have not viewed your blog feed ever as a subscriber.
NewsGator will count anyone who is currently subscribed to your blog even if they have not viewed your blog feed ever as a subscriber.
Pageflakes will count anyone who is currently subscribed to your blog even if they have not viewed your blog feed ever as a subscriber.
Rojo will count anyone who is currently subscribed to your blog even if they have not viewed your blog feed ever as a subscriber.
Yahoo requires that the subscribed user has viewed (logs into My Yahoo or Yahoo Mail) your feed within 30 days to be counted as a subscriber.
Credit:
Feed Tracking Numbers