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,
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.