Facebook publicly released their new ad platform, based on the concept of fan-sumers – facebook users promote the brand (or their usage, purchases, satisfaction, etc , with the brand’s products) to each other.
A good review:
(skip to the facebook section – the myspace section briefly discusses the new Myspace ad platform which is a more generic play around matching ads contextually with users profiles)
TechCrunch Live-blogged the announcement here (and there are many embedded links of interest as well):
http://www.techcrunch.com/2007/11/06/liveblogging-facebook-advertising-announcement/