The problem is that for an informed opinion, your social graph should at least rate a few of these things and assuming Dunbar's Number as the number of relevant follows you might reasonably have, if you reduce that by shared interests you quickly fail to find any relevant opinions. What are the chances that any of them has been to that other country, to that other town and visited any of those 10% of still available hotel rooms you can find for your travel dates? Or even more generic items like headphones ... how many of them have tested headphones that are still on the market in your town ... and taken the time to write reviews?

Google has the social graph and uses it on the few occasions there is a match but it's few and far between.