So apparently the app works because you can access someone's facebook page with the URL www.facebook.com/"#". With the number being the numbered profile they are.
Its curious, however, that Zuck is actually number 4 (www.facebook.com/4), numbers 1-3 are missing. I wonder why?
Source, Dustin Moskovitz: 1-3 were accounts used to test registration. Evidently it only took Mark 3 tries to clear the bugs. Link: http://qr.ae/NMpX6 (Does not require login)
Click on the picture, it links to "facebook.com/4" (=zuckerberg), etc. There are several gaps in between the first 50 ID numbers. Probably some early test/fake accounts as well as people who deleted/deactivated their account. Eduardo Saverin (co-founder) has id 41 and Joe Green has 42.
Its curious, however, that Zuck is actually number 4 (www.facebook.com/4), numbers 1-3 are missing. I wonder why?