If you look at your Facebook profile page (and you didn't opt to change the URL to your name a few years back), the URL is probably followed by a long string of numbers:

MORE FROM BUSINESS INSIDER:
Where Are Facebook's First Employees?
The Soon-To-Be Groupon Billionaires
Groupon Execs Who Have Cashed Out

---

http://www.facebook.com /profile.php?id=5500957

That's likely the order* in which you signed up for Facebook.

When Facebook was founded, every user was given an ID code. It began with ID #0, which redirects every user to their own profile page.

ID numbers 1-3 were likely tests that Mark Zuckerberg used and then deleted; the profiles no longer exist. The first real Facebook users start at ID #4.

Naturally, the first few people to sign up for Facebook were Harvard students. Some shared classes with Mark Zuckerberg. Others were roommates who helped him launch the site.

Zuckerberg's friends made up most of the ten first Facebook sign ups, after which the social network expanded to friends of friends across campus. Eventually it swept all Ivy League colleges, the nation and then the world.

Out of the first 50 profiles, only six are women. The first woman, Sarah Goodin, joined with ID# 33.

We found all of the first members, searched their friends and found out how they were all first introduced to Facebook.

*A source close to the matter tells us user IDs were somewhat sequential but they were chunked up into per-school blocks up to 100,000 IDs. Harvard was the first chunk (0-99,999). My number is about 5,600,000, so its likely that Syracuse was roughly the 56th school on Facebook.

It's unclear when Zuckerberg opened up Facebook to the second school, so beyond those initial Harvard registrations, the number string following URLs may not be precise signup numbers, especially if a user joined Facebook outside of a college network.