As DOS says, finding someones IP address can be done using a vast array of methods.
When you send a 'request' ( Access a webpage it takes note of your IP address ). A IP address is your geographical location which is needed when sending a request to a website otherwise how will the designated packets get to your Router or the Web Servers IP. What you can do with a IP address is scary ( Correct me if I am wrong, Dos ) one of the first things you can is track the IP address. There is even webpages out there offering such a service:
https://www.iplocation.net/Please note: This only tracks one person to a 'Post Code' and even saying that goes a bit far. Obviously they are a array of tools you can use to identify where the person exactly lives. As well as taking notes of your privacy you can 'DDOS' someone aka flood there Network with traffic ( Illegal again, but just another method of attack.
They are other various things you can do with a IP address, get access to the Router and view traffic of the Network, steal passwords and such.
You can avoid your true IP address being found by using various pieces of Software. The most known one is 'Tor Browser'. Which is like a ball of rubber strings, it encrypts your IP address making it look like you had never accessed such a page but someone else has. ( Under a different IP address ) One such disadvantage though is obviously the connection is somewhat 'slow'. Another element with what you can go with is a Virtual Private Network which again encrypts your IP address so your accessing Web pages from a different IP. Such services like Betternet for Windows, HidemyIp address software are prime examples.
https://www.torproject.org/projects/torbrowser.html.enhttps://www.hide-my-ip.com/Please Ayecappy note this: No Facebook account is secure! Never! It is a well known fact Facebook itself track which websites you go on even if you are not logged into it at the time. There is several weaknesses to the page, itself.