Hardly 48 hours after announcing log out page ads, Facebook has debuted a first iteration of its new ad unit: a giant Bing home page that appears once you log out of the site. We haven't been able to reproduce the ad ourselves, but a TechCrunch reader encountered the ad, which only appears once you've clicked the log out button. The Bing search box is fully functional, but when you type in a query and press Enter, you're taken out of Facebook to a separate Bing search results page.

At its Marketing Conference Wednesday, Facebook explained that over 37 million people log out every day, which offers huge potential for advertisers. The new log out ad unit is an option inside Premium On Facebook, a new paid campaign businesses can sign up for that also includes News Feed ads and Mobile app ads.

So why Bing as Facebook's first Premium advertising partner?

Microsoft made a hefty early investment (which is now worth more than $1 billion) in Facebook, and even powers web search results that can appear inside the site's internal search bar. Facebook's new ads likely won't bother users (since they've already logged out), and will provide a new opportunity for advertisers to turn the log out page into the coveted back page of a newsstand magazine.