I think the incentive compounds the self-selection bias. For someone who is an expert in say audio equipment, a small monetary reward isn't going to encourage

I think the incentive compounds the self-selection bias. For someone who is an expert in say audio equipment, a small monetary reward isn't going to encourage him/her to post a review (assuming they have a decent amount of disposable income). However, for those who could greatly benefit from that incentive, they'll post reviews like there's no tomorrow. Another great example of how Amazon has succeeded with user generated content is Listmania--genuinely helpful product lists, all provided for free. I don't completely understand the psychology of why these are created, but I value the content greatly because I know that contributors are not compensated for their suggestions.