Well according to a survey of more than 2000 online consumer's, blog readers are strongly influenced by blog content when it came to there purchase decisions. They also found that readers use links on blogs to discover new content more than they use search engines. Same goes for ads, readers trust the ad on their favorite blog more than on social sites.
Another good reason to advertise.
This info is from the BuzzLogic and JupiterResearch survey released in October 2008.