I'm sorry but social proof is ridiculous. It's not a fair system. It can be manipulated. What if someone has no friends, and they like it that way? Also, most people are not honest. Using feedback on a service is great, but on an individual is scary. Why? Have you tried to sell on eBay ever?
Social proof is not asking only friends to guarantee. Also, your family (father, mother, brother) or co-workers can guarantee. If somebody has no intimate social circle he is not a good target for social proof. And eBay is a perfect example of using social proof. Currently, platforms like eBay enable policing and trust among members (and charge a hefty price for that). If we do it with social proof it means that in order to do the trade the other party - selling party- has to show that he has guarantees from friends, family, co-workers. In case that product received from the seller has not been as promised guarantors can be charged again if the product does not have promised features. Or to put it simply if you sell me an iPhone and instead I get Huawei phone I can get the funds back from your guarantors (and you get your Huawei phone back).
History has shown that most of the communities that were below 150 members did not need police, governmental structures since they were able to self-regulate themselves.