Hey All! I'm not sure if you will find this useful; however, I have learned of a simple way to find the IP address of a Facebook User through Private Messages. (I think this would even include mobile IP's).
There are only a few steps for this, and it is very simple. In order to confirm the IP address of the victim you can use a wide variety of websites; one of the many I will use is the link that is provided below. This will provide you with the account details of the ISP, as well as the Geographical location or GPS coordinates of the computer. You can enter the coordinates of your or any computer on Google Maps to determine an accurate location of the computer. If you are after someone specific, this will help you confirm that this is there IP address. (It's kind of scary, but it I use it to ensure that this is the right person to attack- or attempt to attack).
http://www.iplocation.net/
Follow these steps to find the IP.
1). Close all open programs and all tabs (except for your web-browser and Facebook).
This is essential to ensure there aren't many or other connections to your PC that can help confirm the proper IP address.
2). Open up CMD on windows.
3). Type in netstat -an and wait a few seconds/minutes.
After this search, you should have a list of IP's (depending on how many people you are messaging) and talking to.
4). From my experience, the IP address of your victim will be listed as many times as there are messages from the user you wish to hack.
CMD should say, "Established" in the IP of your victim. This ensures that there is a "connection" between the computers and IP so they can communicate- which is your targets IP and the person sending you the messages.
[Optional]
5). If you wish to confirm that this the IP of your user use the link I have provided above. It can provide you with more information to ensure it is correct.
6). Enter your the Longitude and Latitude coordinates on Google Maps to be 100% sure that this is your victim. If you know where the person lives, or has an idea where the person lives, this will confirm that this is their IP and not someone/something else's.
(It's best to be safe when performing an attack on a computer, so you want to ensure that the attack is on the proper computer).
Hope this helps! I'm not sure if there was a similar post or not, but I decided to post it anyway. I don't see why this would not be in the appropriate section. Hope to hear some positive feedback, and professional criticism if I am wrong! 
