Facebook and MySpace have both started to use profiles and community membership to target ads to their member base. Both are likely to suceed but for different reasons... MySpace's massive membership (growing at 5 million a month) and their innovative SelfServe system gives them enormous scale and a local footprint. Facebook is actively encouraging members to subscribe to a brand, so people who actually really like a brand can make a public endorsement of it and share it through their networks, brands can leverage the already established and trusted social relationships.
Here's an extract from the Facebook press release (the full release is here...):
Today, Facebook Ads launched with three parts: a way for businesses to build pages on Facebook to connect with their audiences; an ad system that facilitates the spread of brand messages virally through Facebook Social Ads™; and an interface to gather insights into people’s activity on Facebook that marketers care about. ...
Advertising messages will gain distribution through what Facebook has termed the “social graph,” the network of real connections through which people communicate and share information. When people engage with a business’ Facebook Page, that action will spread information about that business through the social graph.
Facebook's CEO prefaced his recent remarks at the New York ad:tech conference by saying modestly "the next hundred years will be different for advertising, and it starts today."