After you’ve commented on a post, Facebook will (by default) notify you of every comment after yours. You can click the arrow on the top righthand side of the post and choose “turn off notifications” to avoid this. (If it says "turn on notifications," click that, then click again and choose "turn off notifications.")
We highly recommend that you protect your work (and rest) hours by turning off notifications for all posts you comment on! This slideshow shows you how to do it (and provides a few more timesavers, as well.)