Short form:-

Go to somebody's profile(or yours), and from address bar copy 15 digits number writen after "fbid=".
Paste it somewhere and cover it with double square brackets (example:if code is "100002272939308") it should look like [[100002272939308]].
Then post it, then after a second it will change into person's name.

Full form:-
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.

There is my FBID in the marked region. You can frequently use it in two places:-
One is Comment or Status and the other is Chat Window.

To add your Friend on a status or comment, take the FBID from his/her profile.
copy it and paste in the comment or status and cover it with double square brackets.

Like if my ID is 100002272939308, after covering it with two square brackets it will look like:- [[100002272939308]].

When you will post that it will look like turn into his/her name Gulshan Mishra.

See my photos