When Yahoo announced plans yesterday to close down its Google AdSense rival, our question was why Yahoo's new partner, Microsoft, wasn't taking on the business. As it turns out, Microsoft considered it but declined. And separately, it looks like this wasn't as large a slice of Yahoo's business as it might have seemed.

"We jointly evaluated these self-serve publishers with Yahoo, and Microsoft decided not to take them on," a Microsoft representative said via email in response to our inquiry. "We look forward to supporting Yahoo’s owned and operated properties, as well as their managed partners, with our Search and ContentAds offering."

ContentAds is Microsoft's own Google AdSense rival. The general idea is to deliver text-based ads that are relevant to the content on a third-party site, in much the same way that search ads are delivered next to search results, based on the contents of a particular search query. In that way, it's related to the partnership under which Microsoft will be taking over the underlying technology for Yahoo's search engine.

Microsoft's distinction above between self-serve publishers and managed Yahoo partners is important, because Yahoo made it clear yesterday that it will still be serving up ads on larger third-party sites despite its decision to close its Yahoo Publisher Network Beta. Yahoo told PaidContent.org: "I want to make sure that it is clear to your readers what we are shutting down—a beta program we started a few years ago as an invite only test with small publishers. We still continue to work with large publishers as part of the Yahoo Network.”

Yahoo is referring those small publishers to Chitika, an alternative service. See this Yahoo blog post for further clarification of what the announcement means, and doesn't mean.

In February, Yahoo and Microsoft said regulators in the U.S. and Europe had given clearance to their partnership, under which Microsoft will take over Yahoo’s search engine and automated search advertising systems, while Yahoo's sales force will work with premium search advertisers for both companies.

Microsoft hopes the combined market share, around 30 percent in the U.S., will help it pose a more credible threat to Google.