You're looking for a footprint as we call it. To start you would find a few websites using the FB Social Plugin (I'm sure you can manage that manually). Spot the commonalities between their FB comment plugins. Search different variations of those commonalities in separate quotations until one yields ideal results (sometimes certain things aren't indexed or too broad and what not). Now you have your footprint and can you use Scrapebox to build a list.
I'll post a footprint in the near future. For now though it's best you learn to problem solve this type of thing on your own as working online is a lot of "MacGyver."