When I was in my facebook childhood I get wondered when the fb freaks of that time mentions people from mobile device. By my searches I found an easy way to mention a friend from your mobile device. The following is the best way to do it
Step 1 : Copy your friends UID
Eg :
https://www.facebook.com/profile.php?id=00001636774
In this case the UID is 00001636774
Step 2 : paste the UID in the following format on comment box
@[UID:0]
For Example
@[00001636774 :0]
and then comment. Now You're done
NB: Don't Forget to add " : " & "0" after UID.This Is the important part
This method can also use when using PC. But in fb most of the people uses custom facebook user names .
for example : https://www.facebook.com/abhimanyu.ashok
Here abhimanyu.ashok is the Username
Using this user names you can't mention people in comments.You need their UID .Here is a simple method to find anyone's UID
1. Right Click on the Friend's profile name or profile picture
2. A window will appear there you select " INSPECT ELEMENT"
3.Your Browser will provide the UID as HTML code Just Copy That .Simple.
(Click On The Image To see In Full Size)