In Inbox, select the Facebook Post as the "Parent Post" and filter the stream to follow that post- you can read more about filtering by a parent post here.
After you've done this, you can export the stream by selecting the arrow to the right of the stream name and selecting 'Export Stream'. This excel sheet will include some other columns, but one of them is item text.
Another way is to export stream activity from the Stream Library, this will include alot more and you may need to do a bit more filtering in the excel on your own, but it will include the text. The article below explains this more.