Replying to friends in Facebook comments section using @their-name is what most people do, but instead of having them tagged or sending a notification for getting a reply to their comment, Facebook sends notifications whenever new comments are posted irrespective of them being @ tagged or not. 
For example, check the above screenshot, wherein I’ve received notification when people whom I don’t know commented on a photo I was tagged.
But from now on, you can ‘@’ friend on Facebook comments to tag them in comments and the name will hyperlink to their Facebook profiles and they receive notifications for their comment being replied.
Image Credit: Digital Inspiration
This is for sure a much awaited and useful feature launched recently on Facebook.
Thanks Facebook team!