You first need them to send you a message or a notification of some sort from thier facebook account.
Facebook actually forwards users IP addresses to you using thier own mail servers
In the headers, look for the line that says X-Facebook
X-Facebook: from GFYmail ([OTcuODMuMjEuMjM=]) by
www.facebook.com with HTTP (GFYMail);
All you do is decode OTcuODMuMjEuMjM= into ASCII: 97.83.21.23 using Base64
Easy as pie
