Zuckerberg Facebook ID is 4

Yesterday night I was doing my very first experiments with Facebook API (I'm totally not a developer!) when I came across to an example, I tried to see if everything worked and those results came up.

http://cubeupload.com/im/Gz7Kwu.png

Mark Zuckerberg is the 4th user of Facebook.

So I started to wonder who are the third before.