View Organic Referral Traffic from Facebook

Hi Everyone, My client site sells products that help people with a specific medical condition. There are several Facebook "support" groups out there for people that suffer from this condition. We are in touch with a couple of them but we do not have any relationship with several others. Each of these groups will sometimes link to our website for information or to recommend a product, but all I can see in GA is Organic Social \[subdomain\].facebook.com / referral Is there anyway, either in GA or some other tool (Facebook's Pixel?) where we can see which of these groups is sending us the most traffic? Adding utm\_source is not really an option because we only are in touch with a couple (of course we could reach out to others) and it would depend on them always including it in their links. Thanks in advance, I'm mostly a designer/developer with just a basic understanding of GA, so any help is very appreciated. Jim