As you'd probably guess, the methodology doesn't seem that scientific. From Time:
Facebook doesn’t publicize data on exactly how often a user logs in, though you can bet that they’ve got that information. In lieu of that measurement, this app runs through the timestamps on every post in your feed until it reaches the earliest one, which it uses as the estimated date that you created your profile. Users who are extraordinarily active on the site may get an estimate that is considerably later than the actual date that they joined.
But if you want to feel really shitty about yourself, give it a try here.