How to mention a friend in a comment on facebook?

[Manoj]

Add "@" before your friend's name,i.e., "@full name".
When you will insert the "@" symbol before any name it will automatically start showing you, your friends name that starts with the first letter that you have typed. You can also choose from that list of friend's if you don't want to write full name.
When you have selected the friend you want to mention, you can also delete the last name of that friend to make it short in your comment or post. Even after deleting the last name the friend will be mentioned in the comments, posts, etc.