There are many social media monitoring tools that discover the brand influencers on the blogosphere. Many brands use this to identify these influencers and invite them to either review thier product, their services, or invite them to an event so they can review it.
Thus, I found this very interesting blog post by Steve Rubel about the New Influencers. Its a good read.
http://www.steverubel.com/white-paper-engaging-the-new-influencers