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Facebook founder and CEO Mark Zuckerberg is at number four amongst the first few users on the social networking site.
In fact, the first three accounts on Facebook were reserved for testing, leaving Zuckerberg at number four.
According to The Guardian, everyone on Facebook has a user ID number, created when they joined. The numbers aren't strictly sequential: the first few million are segmented by university, so that Harvard users were assigned 0-99999, Columbia users 100000-199999, Stanford users 200000-299999, and so on.
The first ten users on Facebook include co-founders Chris Hughes, Dustin Moscovitz, first non-founder to sign up for Facebook, and Zuckerberg's roommate at the time Arie Hasit, an acquaintance of Zuckerberg from a computer class Andrew McCollum, and a Facebook engineer Chris Putnam.
