In September, we idly speculated on what would happen if Facebook merged with Amazon.com. Now, in an extension to its new ad-targeting system, Facebook may well get all the benefits of such a partnership, without having to give up any control. AdAge reports that part of its new SocialAds will track buying activity on websites, and report to Facebook users' friends what they're buying. Creepy — but lucrative, since Amazon.com shares a cut of purchases with sites that refer buyers. And Amazon.com and Facebook have already teamed up to let users share book reviews.