You’ve probably lived quite happily so far not knowing the order in which you joined (The?) Facebook. And, indeed, you may have absolutely no desire to find out either. But it’s so incredibly simple, that it would be a shame not to know.
All you need to do is log-in to Facebook, click on your ‘profile’ link at the top right-hand side and then look at the URL bar. Now, this won’t work if you opted to use your own name (or any other username) as a unique identifier in here, an option that Facebook has provided since June 2009. It will only work if you’ve accepted the default string of seemingly random numbers in your profile name.
So, the URL should look something like this: http://www.facebook.com/profile.php?id=123456789.
These numbers are actually the order in which you signed-up to join Facebook. You can actually try a little experiment for yourself – delete the string of numbers after the ‘id=’ part, and enter ‘4’. You will be taken to Mark Zuckerberg’s Facebook page.
As noted in Business Insider, it’s likely that the first three accounts were set-up as tests, and then deleted. Hence, Zuck has nabbed the next available number which was 4. And this goes up accordingly, number 5 belongs to Chris Hughes, Co-Founder of Facebook and subsequent founder of Jumo.
And if you have a lot of spare time on your hands, you can take a look through a little bit of Facebook history to see members as they jumped on board. This won’t tell you how many users Facebook has, however, as many people will have closed accounts and some will have multiple accounts.