There is every possibility you might have been using Facebook and don’t know that adding /4 to the end of facebook’s URL takes you to the founder’s page.
Adding 4 to http://Facebook.com/ will direct to Facebook.com/zuck (Mark Zuckerberg’s profile)
Also, adding 5 to http://www.Facebook.com will take you to Chris Hughes profile as well.
This could be that Zuckerberg is user number 4 on Facebook or the fourth person to join Facebook. Same with Chris Hughes he is the fifth person to join.
This is done with your Facebook ID too if you know your ID. Just type
https://facebook.com/ add your ID.
Try it and see!