Facebook knows your IP address when you're logged into your account through its website or its mobile apps. Other users, however, do not have access to this information. Facebook's servers are always between you and the other Facebook users, which prevents users from tracking your IP address through Facebook via either the Facebook website or mobile apps.
Whenever you do anything on the Internet there is a chain of IP addresses that will eventually lead back to your Internet connection. Connecting to a server requires informing the server of the IP address on the other end of that connection. Even services that promise to hide your IP address only route your connection through their own servers, showing their own IP addresses to the places you visit instead of yours.
When you interact with people through Facebook, you aren't connecting directly to them. Facebook acts as an intermediary between you and every other user. You connect to Facebook's servers and send them data in the form of posts, comments or messages. Facebook's servers, in turn, distribute them to the users who are supposed to see them.
Non-Public Access Records
By necessity, Facebook sees your IP address every time you connect. For security purposes it even keeps records of the IP addresses from which you've logged in. However, this is not information that Facebook makes public to its users. The only IP addresses Facebook users can track through Facebook communications are the ones for Facebook's own servers. Facebook can, however, reveal this information to law enforcement officials.
Mobile Isn't Peer to Peer
The only time your computer can see the IP address of someone you're communicating with online is when you're directly connected to one another. Facebook's mobile apps do not use that kind of peer-to-peer arrangement. The apps connect to Facebook's servers to send and receive messages. Consequently, you still don't have access to user IP addresses on Facebook mobile.