I didn’t need a study to tell me that. But like most studies you need to view it in context. Besides giving away your product to everyone’s friend, what’s the best way to influence a buying decision online? There’s no doubt that it’s online reviews, and brand advocates, assuming your a fan of the advocate. As an example I am very likely to consider a product, even if I'm not in the market for one if it's mentioned in podcast. As a SEO pro I am very suspect of positive reviews, those tend to be search engine link building a social media optimization efforts. But I do give a lot of weight to negative reviews.
PS: I'm trying to find the original study, it's not easy, can someone please provide a link?