We don't have this functionality in JFBConnect. Depending on which social network you'd want, this may or may not be exposed by the APIs. For instance, I see that Facebook has a way to retrieve a single comment by an ID. However, I don't know how easy you can get a list of all IDs of comments for all pages on your sites; if you can, it still won't be an easy undertaking. I don't know how feasible this even is from each social network.
-Melissa
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