The first three accounts on Facebook don't even exist.

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.

In September 2005, high school students were allowed to join and the system changed, and changed again a year later when the site was opened to everyone.

But that means that the very earliest users of Facebook are marked out by their single and double-digit user ID numbers. Some have left the site, but a lot are still there today.

Zuckerberg may have been first on the site, but he's not number one. The first three accounts were reserved for testing, leaving Zuck with the number four.

Straight after come his co-founders, Chris Hughes at five and Dustin Moscovitz at six. Hughes was Facebook's first spokesperson, and after leaving the company, he ran Barack Obama's online campaign in 2008.

His left-wing credentials thus bolstered, in March 2012 he bought long-running political magazine the New Republic, and as publisher and editor-in-chief he has revived its public profile (as well as provided a handy cash injection).

Moscovitz worked on the technical side of the company, and after he left in 2008 he formed a new start-up called Asana, which creates task management software.

And what of Eduardo Saverin, the third of Zuckerberg's co-founders, whose famous falling-out with the company was the subject of David Fincher's film The Social Network? With an ID of 41, he's not even one of the top 20 earliest members.

Long before Saverin comes Arie Hasit, the first non-founder to sign up for Facebook, and Zuckerberg's roommate at the time. After graduating from Harvard, Hasit moved to Israel, where he is currently studying to become a rabbi while working for as a student Rabbi for NOAM, a Jewish youth group.

Facebook's next member – at least of those still on the site today – was Andrew McCollum (ID: 26), who knew Zuckerberg from a computer science class they both took. McCollum currently works at the venture capital firm NEA, where is title is "entrepreneur in residence". As with many of the early members, he also had a role in the creation of the site itself. Among other contributions, he created the very first logo for "thefacebook", as it was then known: a blue and white pixelated image of Al Pacino.

But not everyone with a single or double-digit ID was there in the early days.

Chris Putnam joined Facebook as an engineer in 2005, after he and two friends wrote a computer worm which spread through the site, infecting users' profiles and making them look less like Facebook and more like MySpace.

When Facebook tracked him down, rather than contact the police they offered him a job. But old habits die hard: after Putnam joined, some of the site's engineers realised that the lowest ID numbers were unclaimed.

And that's how he now has an ID of just 13.

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