I think the problem is that if you make it into a business it messes up all the incentives, but if you don't make it a business there isn't an incentive to put time and effort in to combat spam, lawsuits, and so on. Maybe a big open source undertaking, like Wikipedia but with reviews?
I'm genuinely surprised that Facebook hasn't picked up the idea of allowing user-submitted reviews of products, services, places, organizations, etc. Then you could search your friends, or friends-of-friends, for reviews. For Facebook this would be a wealth of information because it indicates which products you actually own and whether you like them. How is that not a huge wealth of information for an advertiser?
It'd be extra good if we could take Facebook out of the equation, but the social graph there is probably the largest currently in existence.
It wouldn't be good for an ad platform to show an ad for something and then show the prospective customer the debunking of said ad.
Facebook makes their money on the market being less efficient. If the market was super efficient and everyone bought what they wanted and was satisfied with it there would be a lot less need for advertising.
My desire for web-of-trust reviews to exist made it harder for me to think as a ruthlessly profit driven company thinks.
Wikipedia is certainly not immune from manipulation. A friend of mine worked for a firm that edited Wikipedia pages for a fee and typically catered towards the ultra-wealthy and politicians.
Spam is automatically taken care for you too (if you ignore the non-review posts).