All of the major feed reading services report subscriber numbers. When most web-based services request the latest content from a feed that’s been burned through Feedburner, that request goes through Feedburner. The request includes a report of the total number of users who have subscribed to that feed. (The exact way each feed reader counts total subscribers varies by service.) Feedburner keeps track of the number reported by each web-based service for each version of the feed and totals them up.

Standalone aggregators do things a little differently. They don’t have overall subscriber numbers. Instead, they use a number of factors to determine how many people are requesting a given feed in a day. For instance, these services can determine if 10 people have requested a feed or one person has requested a feed 10 times. Feedburner has chosen to tally up the number of requests reported by standalone aggregators once every 24 hours (which is why Feedburner stats are updated once a day).

Why do the numbers fluctuate?

Standalone aggregators are the primary reason you generally see lower numbers on weekends. Lots of people don’t turn their computers on over the weekend (although neither Rick nor I knew who these people could possibly be), so desktop clients such as Outlook on those dark computers aren’t requesting feeds.

Other reasons exist for variations. For instance, a service may not report numbers one day for some reason internal to that service. Today is great example of this. Google Reader, for whatever reason, didn’t report subscriber numbers for Thursday, which meant Feedburner couldn’t include them in the nightly tally. On first glance, it appeared that something was wrong with Feedburner, but Feedburner was reporting exactly the same as always — summing up the numbers reported from all services. If you see a big drop, you can look at break out by service (click “Subscribers” under Feed Stats”) to see if any are missing.