When you post something somewhere, or send a file or email, the computer has to know both the SOURCE and DESTINATION of that transmission. What that eventually boils down to is the MAC address on the network interface on the source and destination computers. With a MAC address, it isn't all that difficult to track down a physical location, which will usually give you the owner. However, most people have FAR more information available, so there may be dozens of ways to track them.
You need to realize that everything you do online is "in public." Even if you use encryption, it can still be determined that you send and encrypted message from one place to another, even if the contents of the message can't be accessed.
Just like being in public, you can never really be sure IF and/or WHEN someone is looking at you. Act accordingly.
-Troy