FeedBurner just published a report on the current state of RSS and Atom syndication formats across a variety of publisher types with varying goals. The graphic above helps illustrate the difference between feed search and blog search. While the two used to be almost one and the same, blog search focuses on just blogs and podcasts and not commercial publishers, web services, watchlists, and other peer-produced content.
I think the report was a bit light on monetizing subscribers, so I hope FeedBurner addresses advertising in syndication feeds as its next market report topic.