One non-intrusive way of following a post and its comments is to “like” it. This will give you any notifications of its changes. However, if you don’t want to make it known you are following a thread, click the small gray down arrow in the upper left corner. This will prompt a small dropdown menu beneath it. The first option, “Turn on Notifications” is what you want. This will have the same effect as commenting or liking, and will send notifications to you upon any change or update in the thread.