In September 2015, a rumor started circulating via Facebook that users of the social network could check to see if their accounts had been hacked by commenting "@[4:0]" on photographs. If the code turned into the name "Mark Zuckerberg" when posted, that supposedly meant the posting account had not been compromised:
While typing @[4:0] on Facebook will often result in Mark Zuckerberg's name appearing (results vary depending on operating system), this is not the result of a Facebook security check. This trick has more to do with code, Facebook's mention system, and Mark Zuckerberg's site ID than it does with hacking.
In 2011, after a similar trick (explaining how a long string of numbers followed by the @ symbol could turn into the words "I'm Gay") went viral, Today.com reporter Rosa Golijan explained that the numbers following the "at" (@) symbol simply corresponded to the ID number of a particular Facebook user or Facebook page:
The "@" symbol and surrounding brackets instruct Facebook to convert the ID number into the page's title in a comment or Wall post.
In this case, since Facebook founder and CEO Mark Zuckerberg's Facebook ID number is 4, the "4:0" corresponds to his account and is replaced with his name when inserted into a Facebook post.