From the Feedster website:
Feedster Redefines Internet Search, Targeted Media, and Advertising Delivery
Feedster is a rapidly growing Internet search engine and advertising network that provides timely and meaningful information to consumers and large Internet sites in need of targeted media. Feedster provides a fresh index across over 11 million feeds several times per hour, adding millions of new documents daily. Feedster benefits from the ways that blogging is changing the Internet's basic building blocks, from unstructured web pages to structured documents.
As blogging became mainstream in 2004, it was accompanied by RSS. RSS, which stands for Really Simple Syndication, is the structured data format used originally to syndicate blogs and news, but is now spreading quickly to mainstream commercial use in job postings, e-commerce, and enterprise applications. For instance, over five thousand new job postings are added to Feedster via RSS daily. According to Pew Internet and American Life Project's recent report, 32 million Americans read blogs, 8 million have their own blogs, and 6 million have RSS readers. RSS is growing at a scale comparable to web pages in the mid-1990s.There are now millions of feeds, up from thousands two years ago...
See also: "Blog Aggregation Breaks Out", by Larry Greenemeier, InformationWeek