Initially, when Facebook was developed Mark Zuckerberg tested the numbers 1, 2, and 3 and then created his own ID and assigned it number 4. Therefore, there is no id that has been assigned the number 1, 2, and 3, because they were used for testation purposes. There are different categories of the numeric ids that it allocates. For example, the numeric id pattern of the individual FB user is different from the brand category page. Even the social networking site also assigns an id to every single image, audio, or video you upload.
Find my Facebook ID works to helps the user by looking at the FB profile or page and decodes the numeric id for the ease of users. It let them find the Facebook user easily without going through all the profiles manually.