I have seen lots of posts on Facebook recently with comments along the lines of "Following this", or even just a stream of comments with just the letter "F".
The purpose of these comments seems to be so that the commenter can be alerted to any further comments and discussion on this post.
So, if you weren't already aware of the right way to do it, it's simple. Click on the menu for the post. This is usually displayed as 3 horizontal dots at the top right of the post.