Facebook messages give you a private way to communicate with others on the social networking site. In addition to friends, any other user on Facebook is allowed to send you messages by default; use the "Privacy Settings" menu if you want to limit the ability to send messages to only confirmed friends. When you receive an incoming message, responding is easy--send a message back immediately, or return to the message later and respond.
Go to the "Messages" tab on the Facebook homepage, or click the "Messages" icon on the left side of the Facebook banner. The Facebook banner runs horizontally across the top of every Facebook screen.
Find the message you want to respond in the messages application. Your messages are listed chronologically, with the newest messages at the top of the screen. Click once on the name of the friend(s) involved in a message conversation to open the full message.
Put your mouse in the white horizontal field just beneath the bottom of the current message.
Type your response and hit the "Enter" key to send it. Check the box next to the icon of a cell phone to send a copy of the message to the recipient's mobile device. The copy can only be sent if your friend has enabled mobile services in her Facebook account.