According posts on Facebook, they have claimed to let you know whether your account has been hacked or not. If it shows “Mark Zukerberg”, it means your account is not hacked and otherwise. This is not a trick neither a debugging script. It is not its first time of occurrence. It has happened years back and even last year.
Conventionally, Facebook allows users to tag friends by typing @ proceeded list of friends names. Every user on Facebook has a unique ID. For this case, when you type @[4:0] it tags Mark Zukerberg showing that he is the 4th ID in the database. This is bugs in the Facebook tagging system. When I tried changing @[5:1], it shows Facebook Co-founders “Chris Hughes, @[6:1] shows “Dustin Moskovitz”.
What about if yours didn’t show Mark Zukerberg? does it mean that your account has been hacked? In a normal sense, if your account gets hacked by some dutch bag somewhere. I bet you wouldn’t login to check if it’s working or not.
You can try this with your own Id. Click on your profile and identify your id through the URL.