If I understand correctly, there are a couple things I can suggest for you. :) First, there is this Facebook Ads article that goes over how to view the likes/comments made on the advertisement in Facebook.
In regards to apps for Shopify, and more of the shop integration, you can check out FbComments to add the option of allowing customers to comment on your products directly via Facebook, or you can also check out Credibly to use your Facebook posts, comments and ads, as social proof throughout your online store.
Hope that helps! ^_^