Additionally, the content is sometimes resyndicated -- "used in different ways, mashed up, and spliced together," Lunt says. "A lot of times publishers are happy about that; but other times, they're not and they want to be aware of it. You as a publisher want to know how your content is being redistributed," for example, via a social bookmarking service like Delicious or as content in a blog.

Since there are many ways to access feeds, each with important implications for publishers, FeedBurner uses software to track every request for a feed (as of January 1, the company had just over 170,000) and divides them into five categories.

The most common way people read feeds, Lunt says, is on a desktop with a reader that checks for feeds, in some cases up to 24 times a day. In this scenario, a publisher needs to know that its feed is being accessed by one person, albeit 24 times.

The second way to read feeds is through online blog aggregators, such as My Yahoo or Bloglines, which request a feed only once per update cycle -- but on behalf of thousands of users. FeedBurner tracks the number of users, not the single request.

The third way is through a Web browser, where a person may see a feed, but not subscribe. FeedBurner also tracks "bots" that crawl the Web and add feeds and other content to search engines' indexes, without any human actually reading the material. And the final category includes e-mail feeds that are much like newsletters.

Lunt says getting a breakdown of these different types of feeds is important for FeedBurner clients, who have been experimenting with different ways to present feeds to their subscribers, based on the numbers. For instance, the Houston Chronicle has moved toward categorizing its news feeds into topics such as Hurricane Katrina, Lunt says, as opposed to sections, such as business, sports, and national news.

As for tracking what happens to the content after it's been read, Lunt explains that FeedBurner embeds a transparent picture file the size of a single pixel into the feeds of publishers that pay for the service. Feeburner is notified when a browser makes a request for that tiny image, and the aggregated information is passed on to clients. There is no single way to use this resyndication information, Lunt says: "We're giving them the tools to tune their content, offer more or less [content], and test effective kinds of monetization."

As it becomes easier to track the eyeballs of feed subscribers, and especially as feeds become even more pervasive, RSS will become ripe picking for advertisers. Hundreds of new feeds become available every day, and Microsoft has said its new operating system, Vista, will include feed-reading software, giving the masses an easy way to subscribe to content.

Says Troy Young, the "chief experience architect" at Organic, a digital marketing agency in San Francisco: "I think [feed advertising] absolutely will be huge."