Google's ad relevance was not always what it is today. As marketers got better and the algorithm improved, the ads gained relevance to the user and became more valuable. In facebook's case, marketers will have to figure out how to create ads that are relevant to the News Feed context rather than the search context, but I don't think that the social context is inherently unusable for ads -- it just hasn't been figured out yet.
Facebook has no such mechanism and that is it's big problem. IMHO