First you got to find out the IP address of that User.
To Do so we will be using “netstat” command in windows. If you want to know the IP address of a specific person on facebook or orkut or any chat service, there is only one way: Just invite or ping him for a chat and while chat is ON open ‘Command Prompt‘ on your PC (Start >Run>cmd).
note: before trying this make sure you close all the other tabs in your browser. and only facebook is open. also if possible delete all the history and cache from your browser.
When command prompt opens...