Yahoo! really does want you to make their homepage the center of your web-browsing universe. Next year, they're going to make that easier by integrating Facebook. Not only will you be able to see your network's activity directly on Yahoo!, you'll be able to easily share content on Yahoo! - including answers, news, sports, etc.
500 people visit Yahoo! every month while 350 million people are members of Facebook. Looking at the numbers alone, this is a no-brainer.
BTW, it is quite reminiscent of the new design strategy by MSN - where they've integrated Facebook and Twitter into their new look.