Costolo said that a major challenge for RSS advertising is the diversity of software used to read feeds. "Unlike in the browser world, where supporting three or four browsers gets you 99.99 percent of browsers," he said, "in the feed reader world, we've catalogued over 640 of these things." In fact, FeedBurner supports more than 300 different RSS clients that have more than 1000 users each. Costolo said there are many more clients that are used by a few hundred people. Costolo said FeedBurner also is working with Kanoodle, an ad network that provides search keyword, contextual and behavioral targeting. Kanoodle recently began serving ads into blogs published via Six Apart's hosted TypePad blogging service. Costolo said the ad- and feed-serving systems needed to be adjusted for feeds so that when an ad is rotated it doesn't appear as updated content.