We use the facebook comment plugin to have comments on our site.
To moderate those, we use this tool: https://developers.facebook.com/tools/comments
However,
This code queries for all comments xids used by your application:
https://graph.facebook.com/fql?q=SELECT fromid, text, id, time, username, xid, object_id FROM comment WHERE xid IN (SELECT xid FROM comments_info WHERE app_id = {0}) ORDER BY time desc&access_token={1}&format=json
I'm considering doing the same thing- grabbing all user posts for my application. I'm assuming it's a similar task.
For given user, I will scroll through feed and home looking for app posts. For you, you'd go:
`home?fields=comments`
`feed?fields=comments`
And check for "type" and "id" to match your application.