I'd divide the possible filtering processes into three approaches.
* Throw all your questions to all your friends and all the review sites you "trust". This means any third party site you trust gets a rather excessive ability to spam you - even a site with good user reviews on them are trying push pop-up windows on me. Just because I've gotten good info from X once doesn't mean I want any more from it.
* Throw friend recommendations and trusted sites to a third party "meta-filterer" who organizes things for you. You'd have to really trust that site and essentially there's no reason they'd be better than Google.
* Do the filtering yourself. Most people essentially do that now. I'm working on a project to create tools to automate and improve this this process. Create your own relevance and topic-weighting system that adaptively filters all the other filters. I believe that this kind of approach eventually going to be needed. Not so much because each person can or should do all their topic-relevance-weighting but because this approach would keep the other systems honest.
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Eventually, people are going to realize that both their social graph and their content-relevancy-weightings/algorithm are far too personal to farm out unquestioningly to a third party. The present social networking system is like AOL-email in 1992 except with the added provision that provide can look-at and alter your emails as part of the service. I'd envision this stabilizing to the present email system where webmail exists but has to work more or less the same of email to your personal computer.
Your best bet ends up being to simply include everybody, which improves your chances of finding an expert opinion, regardless of the obscurity of the query.
Logically speaking, no it doesn't. Why couldn't a search engine be aware of topic/product areas? Many already are.