The purpose is to utilize valuable opinions, comments and suggestions by the subject experts, to extract findings and future research directions as well as to report the conclusion to the research community.
What do you mean by "documenting"? If it's retrieving them, then a simple copy-paste to Word file or NVivo database should do -- this is very cumbersome for large groups, so people have developed some tools you can find by googling 'facebook scraping -emails' (it's not the email scraping you want). Anyway, Facebook seems to discourage this type of activity -- not sure if it's in fact against their TOS -- of which you can read more here: https://www.facebook.com/help/community/question/?id=10201290977937615
If you meant documenting in terms of citations, then you can store each comment to include the source (e.g., Person A, Person B, etc.). You can then use these source codes when referring to particular citations.
Thanks for the contribution and valuable suggestion for documenting Facebook comments. You are right I was looking for the way to document comments in terms of citations.