So... using the built-in FB integration, you can't "like" comments, only posts. Using the FB app from the Store, however, you can; there's a little "like" word under each comment which you can tap.
While I had dropped the FB app for the built-in integration, it looks like I'm going to switch back to the FB app with today's update. It's much faster and is better looking than either iOS's or Android's FB app.