I will not discuss it too long but I will tell you the main topic what you came for.
As you know, to create ID on Facebook you need an email ID on Gmail, yahoo, rediffmail or any other host. That host provides you primary ID. When you login to Facebook you with email ID, that ID is the primary ID.
Facebook uses that only for login purpose, but, it officially uses a FBID, which stands for Facebook ID. According to Facebook you are an object, and not only you everything on Facebook is an object, and each object has a unique ID.
This ID can be used by both users and Facebook. If you somehow got your or anybody's FBID you can refer him/her.
As you know, to create ID on Facebook you need an email ID on Gmail, yahoo, rediffmail or any other host. That host provides you primary ID. When you login to Facebook you with email ID, that ID is the primary ID.
Facebook uses that only for login purpose, but, it officially uses a FBID, which stands for Facebook ID. According to Facebook you are an object, and not only you everything on Facebook is an object, and each object has a unique ID.