This point doesn't apply very well to this case. The problem is that there was a lack of verifiable people on Yelp posting negative reviews. Yelp has many more semi-verifiable people on it (i. e. people who've left multiple reviews) than the typical person has friends with experience with a given rarely-used service.

If in this case the problem were likely to be solved by acquaintances, it would also be likely to be solved by Yelp, under its assumption that people need to leave multiple reviews to be considered credible.