Facebook has started selling huge ads on the page that the user sees when he logs out from his account on the network.
Microsoft is the first advertiser to use this service and they are promoting Bing through it.
As a result, you are going to see a major Microsoft Bing advertising if you log out of your Facebook account.
Microsoft is of course also an investor in the company and is one of the partners as well.
Facebook uses Bing search on their platform in addition to Bing Maps as well.